Both endgame and middlegame are one word each in chess literature, but the tag for endgame is currently end-game. Is there an easy way to fix this, or do we have to un-tag everything that has endgame right now and then re-tag it?.
The reason that tag synonyms don't work is that "endgame" is considered to be an alternate spelling of "end-game" so nothing can be tagged "endgame" and the tag synonym cannot be created until the "endgame" tag exists.
N.B. I created a middlegame tag in order to avoid this problem with that tag.