A Theft of Ivory (Mastadon Tusk drop)

When 'The Devouring Diadem' consumed a corpse of one of the bosses the Mastadon Tusk would not drop

for consistency I have killed 'Iktab, The Deathlord' first every time and without the helm equipped the tusk dropped both times, while with it equipped it didn't drop a single time (around 4-5 kills)