I have the same issue right now...trying to find a solution.

Since it's a common issue with other games as well, people have made workarounds you could try, like this one:
https://github.com/scawp/Steam-Deck.Auto-Disable-Steam-Controller/issues

Though idk if it still works.

There's also a Decky Loader plugin for it, which might be more convenient to install but you need to have Decky Loader installed:
https://github.com/TacticalLaptopBag/decky-controller-dock

If you don't have Decky Loader installed yet, i highly recommend doing so:
https://decky.xyz/

It's kinda like a mini app-store within the Steam Deck's game mode full of handy community made plugins and tools for the Steam Deck. Afterwards you can just search for "Decky Controller Dock" within Decky Loader and it installs it for you.

Bump for visibility, this is a game breaking bug for couch co-op.

The only thing that worked for me was pressing B in the Steam Deck controller before selecting any character. The "fake" player goes to P4 place and it let me start the game with just P1 and P2. But I wonder what would happen if 4 people wanted to play coop at the same time...
For sure, this is a breaking change that should be addressed :(

Having the same issue. Weird thing is that when I first played with my kid, it wasnt doing it and then when I tried again it always has the 'third player'. Not sure why it would have not sensed the steamdeck controller the first time.

the decky plugin seems out of date and not available anymore.
dunno for 4 but can t play with 3 controllers as the deck switch to player 4 and cant be disabled from there :(

引用自 Ilriath：

the decky plugin seems out of date and not available anymore.
dunno for 4 but can t play with 3 controllers as the deck switch to player 4 and cant be disabled from there :(

There's an alternative Decky Plugin, called "steamdeck-input-disabler" you could try.

Honestly, i don't understand why Steam just doesn't let you disable the Steam Controller via Steam Input at least on a game-by-game basis or under the condition, that you have another input device connected. One less point-of-failure for game devs trying to maintain good Steam Deck compatibility.

引用自 JellyPuff：

There's an alternative Decky Plugin, called "steamdeck-input-disabler" you could try.

This Decky plugin worked perfectly for me.

not perfect but does the job, thanks!!

Automatically considering every connected controller to be a participating player is a Video Game Sin that should be avoided by any self-respecting video game.

引用自 MrIncognito：

Automatically considering every connected controller to be a participating player is a Video Game Sin that should be avoided by any self-respecting video game.

There's a non-zero % chance for it to be a Steam Deck related issue. For example, i've just recently spend more time than i wanted to trying to get the Steam Deck controller working on another game natively, but trying to configure it always registered "D-Pad Up" inputs (or rather it was called "Hat Switch(U)", which i assume it's related to the D-Pad).
It was never pressed or had been registered in any other situation, except for when the game was specifically listening for inputs through the built-in controller config. Never got it fixed and just made the Steam Deck buttons act like a keyboard as a workaround, which allowed me to bind them to my liking just fine.

You actually do have to press a button to tag in as a player in Vampire Survivors. I've just started party mode with my DS4 plugged in on my Windows desktop and the 2nd palyer wasn't tagging in, until i was pressing something. So maybe it's a similar issue and it's a bug with the Steam Deck controller and in some cases it just sends inputs when games are listening for them.

I'm affected by this issue as well

引用自 luisega：

The only thing that worked for me was pressing B in the Steam Deck controller before selecting any character. The "fake" player goes to P4 place and it let me start the game with just P1 and P2. But I wonder what would happen if 4 people wanted to play coop at the same time...
For sure, this is a breaking change that should be addressed :(

This worked for us.

Have the same problem without a steam deck

I have the same issue and it‘s impossible to play couch coop locally