steam flatpak add drive keeps saying not a supported location for a Steam library folder

[Funk]

flatpak steam wont let anyone add a new library drive anymore, every folder gets bounced with

This is not a supported location for a Steam library folder.

doesnt matter if its ext4 or btrfs, /run/media or /mnt. throwing more flatpak permissions at it reportedly does nothing.

the trick going around is overriding host-root:ro for a sec, add the drive, then remove the override again. works i guess, but handing steam the whole host root even briefly feels sketchy