Partial Stack of wood in inventory. Empty Slot on hotbar. Start harvesting trees. The piece of wood is collected and show on the hot bar, including pieces from inventory. Open inventory and items jump back to inventory occasionally.

Happens when crafting also. If this is a "feature" I kinda hate it. When I am in my tree farm I like to keep an empty slot for saplings that I pick up so I can replant as I harvest the trees.

Cheers.