I have discovered that when you have your inventory open, you are unable to put something away by pressing the hotbar key for that item.

So if you have an M4-A1 on your hotbar as item 1. If you use the hotbar to take that rifle out, when you open your inventory you cannot press 1 and simply put the gun away as you used to be able to. This bug makes the game seem clunky as the second best way to put the item away is to double click it. But that always comes after the instinctual pressing of the number key.