Running into what seems to be the same issue (also on Gentoo Linux with Steam via flatpak). Looking at journalctl shows that an assert inside of Wine is tripped:
Aug 14 12:52:07 selenium steam[5899]: t8.exe: ../src-wine/dlls/ntdll/unix/virtual.c:1907: create_view: Assertion `!((UINT_PTR)base &...