If you mean the update, just download the game again (as explained in the post linked above). Or if you bought the game after this update went live then you already have it
I would also check to see if you might have any other input devices connected that could be sending an up input in the background - a controller with drift, maybe.
This may be the same odd RPG Maker issue one other person reported where the menu key wouldn't work. Holding shift and mashing the key in question fixed it, somehow. I'd try that!
