When I change the category a topic is in (via the edit topic title thingo) I am finding that I cannot also tag. If I try, I get the message “this topic has tags that aren’t allowed” - when this is not the case.
My workaround is to save the category change, and then re-open the edit topic title thingo, and then add the tags. It works just fine then. Neither category limit tags in any way.
This is very similar (or even the same) as:
Can you reproduce it? Happy to do a video if you like.