Mammoth and Summoner Boss don't drop tusks if corpse is destroyed (A2N)

The Mammoth boss didn't drop the tusks for me on two separate tries after I'd destroyed the corpse of the summoner in Act 2 Normal.

I have both Corpsewade and Devouring Diadem equipped. Summoner died first, corpse destroyed, no tusks.
I tried unequipping Corpsewade, same for the corpse being destroyed (this time by Diadem) and still no drop.
After unequipping Diadem, the boss dropped the tusks.