Posted the solution here: https://forum.mendix.com/link/space/app-development/questions/128982
I have had the same issue. I tried all the things mentioned in the multiple tickets about this topic, but nothing worked. The solution for me was to delete all the old styling from our theme folder and to integrate neccessary 'old' styling in the new styling documents. It also resulted in a more logic theme file structure.
Just mentioning this here in case someone else encounters that 'nothing mentioned' works : )