Steps to reproduce the issue

Go to Template Style Site
Open Cassiopeia - Default
Save as copy
Assign Style via Menu or just make it default style

Actual result

No CSS loaded, just HTML on website

System information (as much as possible)

Joomla 4.1.0 - PHP 8.0.10 Xampp

Additional comments

But the alternative way works!!!

Go to Template Style Site
Check Cassiopeia - Default and click "Duplicate"
Assign Style via Menu or just make it default style
CSS loaded, all good!

-> "Save as copy" Cassiopiea Style doesnt load css, but when just duplicated the style the css will be loaded