I think it’s related to this bug and this bug. Basically, if the hearthling drops an item at the moment they put it in place, then the item appears to be in two places at once. The dropped version gets put into a stockpile and appears in the inventory/store lists. But if you try and place it again, they’ll get the original. If you try and sell it, the original is sold from where it is placed in the world.
The trick is to find some way to reliably reproduce the bug so they can fix it.