Magento Grunt Less Compilation behaves weird

I have a custom theme Mycompany/mytheme including <Magento>/<blank> as parent.

The locale is set to de_DE.

When I execute php bin/magento setup:static-content:deploy de_DE everything works fine, my custom-m.less and custom-l.less get compiled into custom-m.css and custom-l.css and I also get the Magento/blank theme files and viewing the magento website with chrome also loads these files.

custom-m.less and custom-l.less are exact copies from the blank themes styles-m and styles-l less files

The Problem:

Using grunt exec:mytheme and grunt less:mytheme kinda behaves strange. I still load the Magento/blank styles-l.css but everything else gets loaded via their own less component files (e.g. _extend.less or _module.less)

Here is my grunt config file:

magento-root/dev/tools/grunt/configs/themes.js

module.exports = { ----------------------------- ... blank and luma stuff here ----------------------------- mytheme: { area: 'frontend', name: 'Mycompany/mytheme', locale: 'de_DE', files: [ 'css/styles-m', 'css/styles-l', 'css/email', 'css/email-inline', 'css/custom-m', 'css/custom-l' ], dsl: 'less' }, -------------------------- ... backend stuff here ... -------------------------- }; 

'css/styles-m', 'css/styles-l', 'css/email', 'css/email-inline' are not inside my mytheme, but as magento devdocs says should anyways be included because of the parent theme inheriting. I also tried to not include these files inside the config file = Same result

BTW: grunt commands throw no errors, seems to work fine, in var/view_preprocessed I see my custom files inside the Mycompany/mytheme folders, same as pub/static/frontend....

I deleted the cache after each new try

Server-side compilation enabled

developer mode enabled

Question: Can someone help me with this? Can someone explain whats wrong?

And what is the expected workflow? Magento Documentation is a bit unclear about that

1. php bin/magento setup:static-content:deploy -> grunt watch ?
2. php bin/magento setup:static-content:deploy -> grunt exec:mytheme -> grunt watch ?
3. grunt exec:mytheme -> grunt watch ?

Should I use the magento php compiler at all? Because I don't want to, way too slow