If you bandolier in a held item (it is on your cursor) the item is placed in the appropriate slot, but the game client UI believes you still have something on your cursor. This prevents you from dragging spells to hotbars, clicking doors, etc. If you click on any hotbutton (as though you were going to swap the button out) the existing button vanishes. I suspect this is pretty new as using a hotbutton to summon and equip my cleric hammers nearly blew all my hotbuttons away when i started clicking things. Confirmed on 3 accounts, each on their own computer, with and without the default UI.
