Edit: Negative resistances are unclear

Original: Jiquani's Machinarium Boss Fire Resistance not rewarded
Either fire resistance is not actually rewarded from this encounter, or the character stat screen is not reporting fire resistance appropriately.

No movement in my fire resistance in act 3 after completing this boss.



Edit: It would appear that at this stage of the game I have an all-round -10% resistance penalty, although it's not at all clear where this penalty came from. Would be good to have a breakdown of resistance sources/penalties on this screen to avoid confusion.

Not a bug and the clarification you're looking for is a pop-up window when you complete an act. Every act completed gives an additional -10% elemental resistances. Pay attention when you kill your next final boss in another act. Should pop up somewhere in the bottom middle of your screen.