Upgrade to 2.1 breaks grunt theme fallback?

Since the upgrade to 2.1 I get the following error on grunt exec:mytheme:

$ grunt exec:mytheme Running "exec:mytheme" (exec) task Running "clean:mytheme" (clean) task >> 231 paths cleaned. Done, without errors. Execution Time (2016-06-27 18:20:12 UTC) loading tasks 217ms ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 70% loading grunt-contrib-clean 4ms ▇ 1% clean:mytheme 88ms ▇▇▇▇▇▇▇▇▇▇ 28% Total 312ms Processed Area: frontend, Locale: de_DE, Theme: Mytheme/default, File type: less. >> [InvalidArgumentException] >> Verify entered values of the argument and options. Unable to get content for 'frontend/Mytheme/default/de_DE/Magento_Theme/css/source/module/_collapsible_navigation.less' >> dev:source-theme:deploy [--type="..."] [--locale="..."] [--area="..."] [--theme="..."] [file1] ... [fileN] >> Exited with code: 1. Warning: Task "exec:mytheme" failed. Use --force to continue. Aborted due to warnings. Execution Time (2016-06-27 18:20:11 UTC) loading tasks 281ms ▇▇▇▇▇▇▇▇ 5% exec:mytheme 5.7s ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 95% Total 6s 

I did not overwrite _collapsible_navigation.less.

This is my themes.js for grunt:

'use strict'; /** * Define Themes * * area: area, one of (frontend|adminhtml|doc), * name: theme name in format Vendor/theme-name, * locale: locale, * files: [ * 'css/styles-m', * 'css/styles-l' * ], * dsl: dynamic stylesheet language (less|sass) * */ module.exports = { blank: { area: 'frontend', name: 'Magento/blank', locale: 'en_US', files: [ 'css/styles-m', 'css/styles-l', 'css/email', 'css/email-inline' ], dsl: 'less' }, luma: { area: 'frontend', name: 'Magento/luma', locale: 'en_US', files: [ 'css/styles-m', 'css/styles-l' ], dsl: 'less' }, backend: { area: 'adminhtml', name: 'Magento/backend', locale: 'en_US', files: [ 'css/styles-old', 'css/styles' ], dsl: 'less' }, mytheme: { area: 'frontend', name: 'Mytheme/default', locale: 'de_DE', files: [ 'css/styles-m', 'css/styles-l' ], dsl: 'less' } }; 

Edit

There is a @import 'module/_collapsible_navigation.less'; in app/design/frontend/Mytheme/default/Magento_Theme/web/css/source/_module.less and I did not copy the _collapsible_navigation.less - but I think it should be loaded via the fallback system from Luma (as it was before the upgrade).

If I copy the module/_collapsible_navigation.less to my theme, it works.

Is this intended?