This is not a Proton issue directly but I don't know where else to put this.
My bad, something went horribly wrong with my Steam Runtime 2.0 installation.
This is https://github.com/ValveSoftware/steam-for-linux/issues/9875.
@kisak-valve Oh, phew, it's not just me then. Thanks.
This is not a Proton issue directly but I don't know where else to put this.