Created attachment 109839 [details]
Flatpak backend looks empty
Discover with the Snap, Flatpak, and PackageKit backends compiled from git master on KDE Neon
When I navigate to the Settings page and click on any list item for a particular backend, Discover takes me to a page that says "Sorry, nothing found..."
Created attachment 109840 [details]
Neon PackageKit backend looks empty
Correction: this only affects the PackageKit backend. Flatpak works.
Created attachment 109939 [details]
Maybe you are trying a repository that doesn't have any packages with appstream information?
KDE Neon sets up multiple repos (like all Ubuntu-based distros) and this reproduces for all of them. I know that there's AppStream info the PackageKit-supplied apps that come from them.
FWIW, this reproduces on my Kubuntu machine, so it's not just Neon. Looks like Ubuntu packaging in general is affected (maybe even all Debian-based packaging, but I don't have a Debian machine handy to test with right now).
Can confirm this on Neon LTS 5.12. Reproduced the same way as OP (flathub works).
Git commit 1ee7ec0f95a90cfdcd4de03c5c44f22dc4a3bea5 by Aleix Pol.
Committed on 01/10/2018 at 15:27.
Pushed by apol into branch 'Plasma/5.14'.
Only allow backends with proper source awareness to filter
PackageKit gives us different ids for the source, so we can't reliably
M +1 -0 discover/qml/SourcesPage.qml
M +1 -0 libdiscover/backends/FlatpakBackend/FlatpakSourcesBackend.h
M +2 -0 libdiscover/resources/AbstractSourcesBackend.h