If you place an item on a armour stand, the stack of items it came from will be stuck in place until that item is retrieved.
For example: I had 16 potatoes and I put one on an armor stand. I then tried to move them out of my hot bar and it did not work. They wouldn't move.
How to reproduce:
-Have 2 or more items in a stack.
-Put one on an armor stand.'
You should not be able to move the items that had the one item put on the armor stand.