

SteamDeck=1 %command%
was the first thing I tried. Marvel Rivals has not a similar problem, because Marvel Rivals supports general Linux and the Steam Deck. The reason why the SteamDeck variable is needed is, because with a recent updated of the game the launcher does not work properly in Proton. And in Steam Deck mode the launcher is not used.
But with Splitgate 2 it seems like they deliberately does only support the Steam Deck itself. So setting a variable to trick them is not enough. At first glance it looks like a similar problem, but only the outcome is the same. The problem is different.
We assume its probably the anticheat, because its a Kernel level anticheat. Maybe it only works with an unaltered image of the Steam Deck OS and Kernel. This is highly speculative and it might be just a bug on the game or Proton side or something. This is still beta and the first day, so they collect information and see what the users have to say. Hopefully they will address this issue. At the very least being open and explain the situation, so we know for sure.