When I try to install a flatpak via a flatpakref file from flathub on a live system e.g. http://cdimage.ubuntu.com/kubuntu/daily-live/current/, I get the following error: "fsetxattr(user.ostreemeta): Operation not supported" This also happens when I try it on the command line. Only when I add the --user option, installing works. So should discover fall back to this mode or just inform the user that installation on a live system is not supported? Maybe this is too much of an edge case, but it could also be useful for testing.
This is an issue in either flatpak or the distribution. Please report there: https://github.com/flatpak/flatpak/issues/