Hello @vyktorjonas, please grab the compatibility report template from https://github.com/ValveSoftware/Proton/issues/new?template=compatibility-report.md, copy it here, and fill out the requested details.
Simply starting the game and pressing start to go to the main menu and you can see the bug
Disabling Steam Input fixes the issue
Nothing extracted yet.
On version 8 of Proton, Sackboy always auto enables a ghost second user/joystick to which is controlled by myself, I play with two characters always doing the exact same thing, I can't choose only 1 player because it's always detecting a "second joystick", if I actually plug in a second joystick, there's then 4(!!) characters on screen, this issue doesn't happen in Proton version 7