Setting the gamescope command should fix that, but give me a second to test it. I may not have a steam deck yet but I do have the same software stack as I’m running the same build of steam.
Tested, seems to be a bug with gamescope and/or steam’s gamepad mode, might be worth opening a support ticket with valve or a bug report on gamescope’s repo at https://github.com/Plagman/gamescope