Not certain this is a bug, but it definitely looks like something not working right.
I go to edit my category, Tags tab. I try to select a tag group for the “Require new topics to have tags from tag groups” option. I can select the group as shown in screenshot, clic Save category, and it silently fails. The tag group isn’t there.
I tried this on an unaccented category and with an unaccented tag group, just to make sure that wasn’t the problem.
Also tried on mobile, same problem.