Holy banana! Good spotting Pent. Well, looks like a bug after all. Pretty rare bug though I suppose we can add this to the list. I'd say the second if statement needs an
Oddly I've been looking at the problem of how fire damage is dealt in an explosion. Each explosion splits the total damage into a fire damage part and a physical damage part. I have found that fire damage from a big mine is around 93, so physical damage should also be around 93. The big mine puts in the biggest hit, beaten only by a blast from a veteran kamikaze dwarf at 112 physical/fire damage. Not sure how much an applied backpack does, maybe around 74.
I presume any character with fire resistance as an intrinsic, will receive less fire damage.