It seems to only happen when hovering over a single item, and then closing the inventory, stacks of 2 or more seem unaffected
This is only true for the local inventory, on external inventories (e.g. chests) it always takes items when hovering over them

Steps to Reproduce:
open inventory
close inventory while hovering over item

Observed Results:
the item moves to the first available slot, if no slots are available it drops

Expected Results:
the item stays where it is