Well, there can also be issues if there are dice rolling in the chat box at the moment that you pick up dice yourself, i.e., the dice contained in the dragdata end up being short a die or end up including one of the dice that was already being rolled. I see this happen sometimes in my "The Box" addon when I try to roll dice through The Box multiple times in rapid succession (which is easier to do using the box due to the way it works).

Maybe this is related somehow, as in you end up with a dragdata object that gets dumped onto the hotkey whenever there are dice rolling already, and that clears the hotkey?