Uh, so I started a new game. Entered the cave that Beodur's in, freed him, Eder and the Watcher leveled up. I proceeded to level Eder first, which proceeded without problems, but when I finished the level-up, my Watcher had been transformed inexplicably into Eder. Tried saving and reloading, but the problem was still there.
I have the save file, but can't seem to upload it.