Warrior with a spear, quest item not dropping from Bone pit boss

Killed the bone pit bosses with a warrior using spear. Killed with rake and rapid assault. Did not drop bone tusk for quest item. Caster killed first. can provide screen shots if needed.

I believe I have found the cause of the bug, if the dev team wants to verify this please do. I am wearing a Devouring diadem. if you kill the mage first on the boss the helm eats the body and causes the encounter to not complete the quest and doesn't drop the horns.