It seems that unequiping something from hotbar while you have an item in your inventory selected is a bug from it thinking by hitting unequip on hotbar, that it's the same as if you did so in the inventory instead of whatever is on the hotbar. What I recommend as a fix is to hold the alt key, and click on a hotkey to unequip that (or one of that from the stack), as holding alt makes hotkey actions act as unequip instead of equip+use.
Edit: can also hover over an icon for a few seconds for the tooltip to appear, and use the buttons on the tooltip, pretty cool that that is somehow not broken.
