TO Save this attribute value to Order address, you need to use Below event:

• sales_model_service_quote_submit_before
• sales_model_service_quote_submit_success

At those events you need to copy the field value from quote address to order address object then, the value will save to sales_order_address table.

events.xml

<?xml version="1.0" ?> <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd"> <event name="sales_model_service_quote_submit_before"> <observer instance="StackExchange\Magento\Observer\Sales\ModelServiceQuoteSubmitBefore" name="stackexchange_magento_observer_sales_modelservicequotesubmitbefore_sales_model_service_quote_submit_before"/> </event> <event name="sales_model_service_quote_submit_success"> <observer instance="StackExchange\Magento\Observer\Sales\ModelServiceQuoteSubmitSuccess" name="stackexchange_magento_observer_sales_modelservicequotesubmitsuccess_sales_model_service_quote_submit_success"/> </event> </config> 

Observer class: ModelServiceQuoteSubmitBefore.php

<?php namespace StackExchange\Magento\Observer\Sales; use Magento\Framework\Event\Observer; class ModelServiceQuoteSubmitBefore implements \Magento\Framework\Event\ObserverInterface { /** * Execute observer * * @param \Magento\Framework\Event\Observer $observer * @return void */ public function execute(Observer $observer) { $order = $observer->getEvent()->getOrder(); $quote = $observer->getEvent()->getQuote(); try { if ($quote->getBillingAddress()) { $order->getBillingAddress()->setAMobilePhone($quote->getBillingAddress()->getAMobilePhone()); } if (!$quote->isVirtual()) { $order->getShippingAddress()->setAMobilePhone($quote->getShippingAddress()->getAMobilePhone()); } } catch (\Exception $e) { // add logger } return $this; } } 

Observer class 2: ModelServiceQuoteSubmitSuccess.php

<?php namespace StackExchange\Magento\Observer\Sales; use Magento\Framework\Event\Observer; class ModelServiceQuoteSubmitSuccess implements \Magento\Framework\Event\ObserverInterface { /** * Execute observer * * @param \Magento\Framework\Event\Observer $observer * @return void */ public function execute(Observer $observer) { /** @var \Magento\Quote\Model\Quote $quote */ $quote = $observer->getEvent()->getQuote(); $order = $observer->getEvent()->getOrder(); if (!$quote->getId() || !$order->getId()) { return $this; } try { if ($quote->getBillingAddress()) { $order->getBillingAddress()->setAMobilePhone($quote->getBillingAddress()->getAMobilePhone()); } if (!$quote->isVirtual()) { $order->getShippingAddress()->setAMobilePhone($quote->getShippingAddress()->getAMobilePhone()); } } catch (\Exception $e) { // add logger } return $this; } } 

Changes at mixin files

From

shippingAddress['extension_attributes']['a_mobile_phone'] = jQuery('[name="a_mobile_phone"]').val(); 

to

 $.each(shippingAddress.customAttributes, function(index, eachCustomAttribute){ console.log(eachCustomAttribute); if(eachCustomAttribute.attribute_code == 'a_mobile_phone') { shippingAddress['extension_attributes']['a_mobile_phone'] = eachCustomAttribute.value ; } }); 

If the first observer is working then do not need to use second observer ModelServiceQuoteSubmitSuccess

I've got a same issue like yours, i've made a Custom Customer Address Attribute but can't save it from checkout, so i made an Event/Observer like this:

<?xml version="1.0"?> <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd"> <event name="checkout_onepage_controller_success_action"> <observer name="custom-address-attr" instance="CustomerAddress\Attribute\Observer\AfterPlaceObserver" /> </event> </config> 

My event name is checkout_onepage_controller_success_action and my Custom Address Attribute Name is customer_tax_exempt_number, after that I made an Observer like this:

<?php namespace CustomerAddress\Attribute\Observer; use Magento\Framework\Event\ObserverInterface; class AfterPlaceObserver implements ObserverInterface { public function execute(\Magento\Framework\Event\Observer $observer) { $objectManager = \Magento\Framework\App\ObjectManager::getInstance(); $addressRepository = $objectManager->create('\Magento\Customer\Api\AddressRepositoryInterface'); $addressFactory = $objectManager->create('\Magento\Customer\Model\AddressFactory'); $checkoutSession = $objectManager->create('\Magento\Checkout\Model\Session'); $orderFactory = $objectManager->create('\Magento\Sales\Model\OrderFactory'); $jsonHelper = $objectManager->create('\Magento\Framework\Json\Helper\Data'); $orderIncrementId = $checkoutSession->getLastRealOrderId(); $orderInfo = $objectManager->create('Magento\Sales\Model\Order')->loadByIncrementId($orderIncrementId); /* NEW WORK */ $bssCustomfield = $orderInfo->getBssCustomfield(); $jsonBssCustomfield = $jsonHelper->jsonDecode($bssCustomfield); $jsonTaxExemptNumber = $jsonBssCustomfield['tax_exempt_number']['value']; $addressRepository = $objectManager->create('\Magento\Customer\Api\AddressRepositoryInterface'); $addressObject = $addressRepository->getById($orderInfo->getShippingAddress()->getCustomerAddressId()); $addressObject->setCustomAttribute('customer_tax_exempt_number', $jsonTaxExemptNumber); $addressRepository->save($addressObject); } } 

This method solved my problem, i hope it'll solve yours too.

To hide the extension attribute in the shipping address, you need to override the file Magento/Checkout/view/frontend/web/template/shipping-address/address-renderer/default.html

requirejs-config.js

var config = { map: { '*': { 'Magento_Checkout/template/shipping-address/address-renderer/default.html': 'Vendor_Module/template/shipping-address/address-renderer/default.html' } }}; 

View/frontend/web/template/shipping-information/address-renderer/default.html

Copy the code here and edit as per your requirement-

<!-- /** * Copyright © Magento, Inc. All rights reserved. * See COPYING.txt for license details. */ --> <div class="shipping-address-item" css="'selected-item' : isSelected() , 'not-selected-item':!isSelected()"> <text args="address().prefix"/> <text args="address().firstname"/> <text args="address().middlename"/> <text args="address().lastname"/> <text args="address().suffix"/><br/> <text args="_.values(address().street).join(', ')"/><br/> <text args="address().city "/>, <span text="address().region"></span> <text args="address().postcode"/><br/> <text args="getCountryName(address().countryId)"/><br/> <a if="address().telephone" attr="'href': 'tel:' + address().telephone" text="address().telephone"></a><br/> <each args="data: address().customAttributes, as: 'element'"> <each args="data: Object.keys(element), as: 'attribute'"> <if args="typeof element[attribute] === 'object'"> <!--<if args="element[attribute].label"> <text args="element[attribute].label"/> </if>--> <ifnot args="element[attribute].label"> <if args="element[attribute].value"> <text args="element[attribute].value"/> </if> </ifnot> </if> <if args="typeof element[attribute] === 'string'"> <text args="element[attribute]"/> </if><br/> </each> </each> <button visible="address().isEditable()" type="button" class="action edit-address-link" click="editAddress"> <span translate="'Edit'"></span> </button> <!-- ko if: (!isSelected()) --> <button type="button" click="selectAddress" class="action action-select-shipping-item"> <span translate="'Ship Here'"></span> </button> <!-- /ko --> </div> 

Comment out the label part and you should be good.

I never got fieldsets to work. I would suggest copying fields using an observer.
Below I've pasted parts of a working example...

First, you just need to specify the AddressInterface and OrderAddressInterface interfaces in the extension attributes (it should also work with just AddressInterface):
./etc/extension_attributes.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd"> <extension_attributes for="Magento\Quote\Api\Data\AddressInterface"> <attribute code="a_mobile_phone" type="string"/> </extension_attributes> <extension_attributes for="Magento\Sales\Api\Data\OrderAddressInterface"> <attribute code="a_mobile_phone" type="string"/> </extension_attributes> </config> 

For fieldsets, create an observer, add an event before the order is placed. https://devdocs.magento.com/guides/v2.3/ext-best-practices/tutorials/copy-fieldsets.html

./etc/events.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd"> <event name="sales_model_service_quote_submit_before"> <observer name="vendor_module_sales_model_service_quote_submit_before" instance="Vendor\Module\Observer\SaveOrderBeforeSalesModelQuoteObserver" /> </event> </config> 

./Observer/SaveOrderBeforeSalesModelQuoteObserver.php

<?php namespace Vendor\Module\Observer; use Magento\Framework\Event\ObserverInterface; class SaveOrderBeforeSalesModelQuoteObserver implements ObserverInterface { /** * @var \Magento\Framework\DataObject\Copy */ protected $objectCopyService; /** * @param \Magento\Framework\DataObject\Copy $objectCopyService */ public function __construct( \Magento\Framework\DataObject\Copy $objectCopyService ) { $this->objectCopyService = $objectCopyService; } /** * @param \Magento\Framework\Event\Observer $observer * @return SaveOrderBeforeSalesModelQuoteObserver */ public function execute(\Magento\Framework\Event\Observer $observer) { /* @var \Magento\Sales\Model\Order $order */ $order = $observer->getEvent()->getData('order'); /* @var \Magento\Quote\Model\Quote $quote */ $quote = $observer->getEvent()->getData('quote'); $shippingAddressData = $quote->getShippingAddress()->getData(); if (isset($shippingAddressData['a_mobile_phone'])) { $order->getShippingAddress()->setAMobilePhone($shippingAddressData['a_mobile_phone']); } $this->objectCopyService->copyFieldsetToTarget( 'sales_convert_quote_address', 'to_order_address', $quote, $order ); $this->objectCopyService->copyFieldsetToTarget( 'sales_convert_quote_address', 'to_customer_address', $quote, $order ); return $this; } } 

Finally, you need a plugin: ./etc/di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd"> <type name="Magento\Quote\Model\ShippingAddressManagement"> <plugin name="vendor_module_shipping_address_management_plugin" type="Vendor\Module\Plugin\Checkout\ShippingAddressManagementPlugin" disabled="false" sortOrder="10" /> </type> </config> 

./Plugin/Checkout/ShippingAddressManagementPlugin.php

<?php namespace Vendor\Module\Plugin\Checkout; use Psr\Log\LoggerInterface; use Magento\Quote\Model\ShippingAddressManagement; use Magento\Quote\Api\Data\AddressInterface; class ShippingAddressManagementPlugin { /** * @var LoggerInterface $logger */ protected $logger; /** * ShippingAddressManagementPlugin constructor. * @param LoggerInterface $logger */ public function __construct( LoggerInterface $logger ) { $this->logger = $logger; } /** * @param ShippingAddressManagement $subject * @param $cartId * @param AddressInterface $address */ public function beforeAssign( ShippingAddressManagement $subject, $cartId, AddressInterface $address ) { $extAttributes = $address->getExtensionAttributes(); if (!empty($extAttributes)) { try { $address->setAMobilePhone($extAttributes->getAMobilePhone()); } catch (\Exception $e) { $this->logger->critical($e->getMessage()); } } } } 

This will ensure that the data is added on sales_order_address table. You can take it a step further by adding a plugin on Magento\Sales\Model\Order\ShippingBuilder, so you can make it available in the order api endpoint rest/all/V1/orders/:id