[Autumn]

Fire Resist is affecting it and that is intended, when it says unresistable it means it goes through phys.DEF and armor, not that fire resist doesn't affect it. Fire Resist affecting the amount is seemingly intended, unless unresistable means a 3rd thing now.

After testing around a bit on one of my mechanations I was able surmise that with 0 fire resistance (I put on a jupiter badge) that I was taking the proper amount of damage.

Though if its doing physical damage, sources like Turtle Shell would reduce its damage, but not your DEF.