I had already tried refreshing (indeed I had reconnected to a new copy of the data source to see if that would fix it).
Any way after much further checking, I found the problem. The field had somehow created an alias that was renaming one possible value (Priesthood Leadership) with a different value (Overseer of Priests)
I've no idea how this happened, but by removing the alias, I solved the problem.
Appreciate your suggestion though, thank you