Uber Elder not dropping loot

I ended up dying to uber elder when the elder was very low health and my siege ballistas ended up finishing him while I pressed the "restart from last checkpoint" button. When I entered back into the instance, zana was doing her animation to kill the elder but there was no elder in the animation. This led to the fight not dropping any items whatsoever.

If you leave the boss arena while the boss is being killed, it counts as if you are not there, and thus the loot will not drop. You should have waited in the arena longer until Zana finished her animation.