If getting a black screen upon launching the game in Flatpak Steam, try to create a new drive in Settings -> Storage and install/move the game there. I've used /home/***username***/.steam. It works because with the default drive (/home/***username***/.var/app/com.valvesoftware.Steam/data/Steam) some game files' path exceed the maximum (254). You might need to grant access to the new folder via flatpak CLI or Flatseal.