At random times, my hotbar items will get randomly switched out with random items in my inventory. Sometimes the items will even duplicate their icons which will take up more slots in the hotbar but will not affect the quantity of the item. For example: I only have a diamond sword in my hotbar with several stacks of cobblestone in my inventory. After a while of going through my inventory and walking around, I find several copies of A single stack of cobblestone. The icon spreads throughout the entire empty hotbar.