The new proton release and Steam runtime (soldier) break on every game start via Steam with the following log in Silverblue:
bwrap: Can't mkdir /usr/lib64/gconv: Read-only file system ln: failed to create symbolic link '/run/user/1000/SteamLinuxRuntime.4297668b2bb5059a/socket' -> '': No such file or directory
I’ve not found a workaround except disabling the runtime yet.
EDIT: Still debugging the issue. Maybe this is not the root cause.
Available workaround: https://github.com/flightlessmango/MangoHud/issues/369#issuecomment-709902078