It's defo something to do with the button under the screen, try holding it down for a few seconds, getting it back to the temp screen, then holding it down again and it should move to bypass, then again and it should change back to wattage.
I think it happens because I finger fire and the fingers below can hit that button under the screen. I got mine back by randomly pressing and holding that button but I don't know exactly what I did.
Yeah, it looks like press and hold the button under the screen until it goes back to showing temp, then 'long press' the button for 3 seconds to switch modes.