Open ANY 10 0r 11 msstyle in WSB and it will show as 8.1 in the title bar. Always been like that.
I'm suddenly getting the same error message as you on a msstyle that was testing fine a week ago. For some reason a shell folder keeps appearing in the Program Data Testing folder that shouldn't be there. I've tried every trick I know to fix this but nothing works including saving first, replacing the shellstyle etc.
The latest beta will show Windows 10 or Windows 11 unless the theme has been modified in a way that causes the version check to get confused. Can you send me the theme that used to work but doesn't work anymore so I can see why it may be failing?