So far I've noticed that :< and !? show up as artist tags for me when they're in the sidebar of the image, but they're still general tags when checking under "Tags" and they show up as general in the sidebar when doing tag searches too. I can't really find anything in common here (looking at a few other tags with colon, exclamation mark, etc., they seem fine), so I'm not sure if there are any others.
Updated by Algasir