The dragon you fight in Act 3 has a fire aura. For some reason that fire aura does damage every few seconds as if it was RTwP, even when combat is set to be turn-based and it should only trigger once per round, meaning that it’s extremely lethal to fight the dragon with turn-based turned on.
To replicate, just start the dragon combats in turn-based mode, and then have someone stand next to it. I specifically remember it occurring in the ambush and the tower combats.
Also the dragon in general seems to be poorly balanced for turn-based, as it can fairly reliably kill an entire party before they can act, but that’s not totally related to the actual bug.