Remnant Boss Fire bonus (+10%) did not apply in Cruel.

I fought the Remnant boss and got the Flame Core. When I pulled up my character sheet, my fire resistance was at 69% (Max 80 due to a +5 from an item) and it didn't go up when I used the item, though it should have jumped to 79%.

Is it possible that this item only works on Cruel if you got it on normal? It's possible I missed the boss in Normal. I'll be checking tonight.

Edit: Nope, I went back and beat Blackjaw The remnant on normal and he didn't drop the item, so I should have both, but it still didn't seem to work the second time.

Same issue, didn't get an increase on cruelty difficulty.