Specific tag cannot be added to posts or tag groups anymore

I have a tag stage-2 that cannot be added to topics anymore. Here is a screenshot of my /tags page:


I want to add the tag stage-2 to a post, but I get “No matches found” instead of the normal “Create Tag”. I tried removing it and re-adding it to the tag group that it was originally on, but got the same “No matches found” message when I tried to add stage-2 back to it.

Bumping this. I need some help on how to debug these kinds of issues. The forum instance is hosted by Discourse directly.