If I go to Filters -> Tags the tag is not there, it only shows up if i right click on a game and go to manage tags.

I have removed it from every game in the library. I have also checked the hidden list which is empty. This is a tag that was imported from Steam.

Is there anything I can do to try and remove it?

tag is bugged, it sometimes takes me 5-6 attempts at adding a tag before it accepts it, then I can use it to filter and add it to other game.