When multiple alien corpses occupy the same space/tile on the battlefield, the dropped items all seem to disappear when a soldier stands on the corpses and opens inventory.
The attached screenshot was taken right after the aliens were killed on the same spot. (3 corpses, actually).
I did not try to reproduce it, but it did happen a couple of times by accident, and each time I noticed there were more than 1 dead body on the tile. (** Often happens on narrow door/pathways.)
On the bug importance scale mentioned by the bug report sticky, I suppose this is a 6 ("A minor feature doesn't work: the application is usable, but it's less fun").
Thanks!