I suspect maybe Asahi fails this check:
https://gitlab.gnome.org/GNOME/gnome-software/-/blob/346da0d84d96657d570964af0b1124ce451216e8/plugins/fedora-pkgdb-collections/gs-plugin-fedora-pkgdb-collections.c#L74
i.e. gnome-software decides this isn't "Fedora", so doesn't activate the Fedora upgrade plugin. It's just a guess, though, I might be wrong. Hector Martin (npub1qk9…azpx) might know for sure.