Side note: I was gonna avoid rebuying PlateUp on Switch because it might not have had Cross Play at the time, but I think it does now.
Spelunky be damned, still doesn't mean I should...

Could be borked Steam Input only on Deck; I'm aware Valve is pushing more controller input updates before they launch their new hardware.
Could be PlateUp; they announced a lot of console code changes last September. Maybe you found a new edge case they didn't fix yet for Steam Deck.

Game name?
Maybe there's known reports on ProtonDB

👀

I am honored?