As the title said, Nihlathak will sometimes hover over to a difficult to reach area, or on top of one of the corners of his platform. If he dies in one of these positions, he sometimes will fail to drop any loot at all.
I’ve been farming keys for the past day or so, and in that time I’ve done probably close to 100-200 halls of vaught runs. I’ve probably seen him bug out drops ~10-15 times.