Update: Appears that the problem is not the elevator, but instead the shape of the ground below it. I assume that if the ground has too much incline or is too uneven for the treasure to land on, it will vanish instead of landing.
I have had a heroic treasure vanish also while falling down from a cliff. It didnt land on the cliff nor the ground below it.
EDIT: Im having trouble replicating the issue for the purposes of a video…