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.

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.

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.

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?

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.