Hi DJ
No one has come up with an explanation as yet, but further to my earlier post, consider this:-
Water molecules contain one heavy oxygen atom and two light hydrogen atoms, so it weighs less than atmospheric oxygen, which contains two heavy oxygen atoms, and atmospheric nitrogen, which contains two heavy nitrogen atoms. These water molecules force heavier molecules to spread out, which reduces the air pressure, a reduction in air pressure causes changes in the drones barometric readings, so is it possible that when the drone took off, there was Less/More humidity (fog conditions at the time) later when you landed the degree of humidity present locally where you were standing changed, and so the drones instruments did not show that it had arrived back at the starting height....? just a guess I don't know but it is a consideration.
Waylander