You can verify whether this is an issue with GNOME Software or your metadata by opening a terminal and running this command:
appstreamcli what-provides mediatype application/epub+zip
If that yields results, it means that you have the metadata but GNOME Software doesn't read ir properly for some reason.
You can verify whether this is an issue with GNOME Software or your metadata by opening a terminal and running this command: epub+zip
appstreamcli what-provides mediatype application/
If that yields results, it means that you have the metadata but GNOME Software doesn't read ir properly for some reason.