Currently, the backpack is a little counter-intuitive for your hearthlings. Hearthlings can carry a maximum of 4 items, but it usually looks like their backpack can only hold 3. When they pick up the last thing, they hold it in their hands while they carry it to the stockpile. So the last item never actually appears in their backpack pane.
Now it’s great that you can actually see the hearthling carrying a thing, so that should definitely stay. It keeps them from looking like they are just wandering around doing nothing. But I do think the backpack pane should include what the hearthling is holding.