Bosses not dropping loot

I've noticed that the skeletal mastadon and summoner bosses are frequently do not have loot drops for me on my poisonburst/toxic growth pathfinder.

I've attached a screenshot of when this occurs. There seems to be some anomaly with the graphics when this occurs as well. That graphic is stuck there and doesn't seem to go away.

It doesn't happen ever time with this boss but it's more frequent than other bosses although I've seen no loot drops on other bosses as well.

Btw, I dropped an Exotic Coin on the ground to show that items aren't being hidden.