Internal CSS not loading after update from 3.5 to 4.4

[k-webdesign]

Affected version(s)
Tested with 4.4.x

Description
When I update an 3.5 installation to 4.4 that contains internal CSS files (e.g. when using a theme), the CSS won't be loaded in the frontend. I need to go into every single internal CSS file and edit any element (just click "edit" and save without changes).

How to reproduce
Update an Contao 3.5 with internal CSS and check the frontend.

[leofeyer]

What about "System" -> "Maintenance" -> "Purge the script cache"?

[Teetrinker]

What about "System" -> "Maintenance" -> "Purge the script cache"?

Doesn't resolve the problem. (and shouldn't be necessary, as the other scripts are automatically in the output)

[leofeyer]

That's strange, because the help text says

Removes the automatically generated .css and .js files, recreates the internal style sheets and then purges the page cache.

and that is exactly what the Automator::purgeScriptCache() method does. Are there any Generated style sheet… entries in the system log?