Placing the helmet in the hotbar causes the player character to repeatedly put it on and then take it off. Helmet does not appear to respond to mouse clicks to manually put on or remove it. It does this in third-person mode in single-player; have not been able to test multi-player.
Helmet remains on if another hotbar slot is selected.
Helmet is removed from player model if the helmet item is taken off the hotbar, but I suspect this is intentional and therefore not important.