Healing magic is different that fire resistance. I can understand a Fire Elemental being healed by fire, but someone of Flesh and Blood should not be healed by Fire, regardless of their resistance to being burned. To heal Flesh and Blood requires a different kind of magic, not negative burning.

And yeah I know it's a single player game and I know I can just make the game difficult on myself, but I'd rather game logic be logical. But to each their own I guess.