DiceLovesBeingBlown said:
It changing into a character tag somehow isn't. Did you check if perhaps someone else just happened to change the type of the tag at the same time as you adding it?

I've recently created two tags ending in _(character), one of which I'm 95% sure I didn't attempt metatag use for, and both came out as character tags.
I assumed there's some form of automation for tags containing certain text.