On steam deck and controller, you can map a button to send a combination or sequence of inputs to the game.

Maybe you accidentally added one of those mappings on your default controller configuration? Check your controller config carefully or reset it.

I suddenly feel so dumb and vindicated at the same time.

I'm using a easy smx controller which form some reason was connecting in Nintendo switch mode to the deck.

Removing and switching the mode to Xbox controller fixed the whole thing, I also wiped any programming just in case.

Why do I feel vindicated?

I kept the instruction booklet.

I’ve had something like this on a Playstation 3 controller a while ago.

It was a problem with some foam which got thinner with age and a contact patch.

I don’t know if the same thing could apply to a few years old Steam Deck, but the symptoms were exactly the same.

Steam should be loading a separate controller config for every game, or at very least loading a default profile of "every button is literally the same as on an Xbox controller". A weird SteamInput config shouldn't affect across all games, I think.

Does it act up in Desktop mode? How about if Steam is closed, and you play something from Heroic?

It's possible that a hardware fault is sending insane inputs. Bad connections to the controller-sub-boards? The controller microchip just going batty?

Sounds really weird. Do you have any deck plugins installed? Does it happen when browsing steamOS menus/desktop mode?

I wonder if it could just be an issue with the default controller with joystick trackpad control config, that might have it affect most of your games. Might try specifically using a different config in some games and see if that changes anything.