MiriOhki: Except that the tag button isn’t doing anything. Whether clicking the right stick or using the middle mouse button, the game doesn’t acknowledge anything. If it wasn’t for the fact that it does work in the system menu to bring up the controller layout, I’d have assumed it was misassigned.
It's rather hard to spot. When you tag them there's a tiny yellow bar appears above their heads, it took me a couple of tries to notice it. The big problem is if you then try to tag them again you actually de-tag them.
Easiest way to check is probably to try tagging one, then put a wall between you and them. If you can still see them, they're tagged.