This is a minor bug and reproducible.
If you drag an item to one of the characters on the bottom part of the inventory screen and drop it there it will be placed on the inventory in the position where it is not visible.
I did this a few times because I forgot that the inventory is the same for all characters, at first I thought I lost the item because it vanished from the screen instead of going back to its original position.
Then when I sorted the inventory I found the missing item.
So I tested it again and instead of sorting I scrolled down and I found the item there.
I guess that since that part of the inventory is not visible, that the item should go back to its original position instead of being put there.
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