Kubuntu 17.10 + Kubuntu backports PPA Discover 5.11.4 Discover doesn't include the Flatpak backend by default in Kubuntu. I installed it (`sudo apt install plasma-discover-flatpak-backend`) and restarted Discover, then browsed the app list looking for apps I know there are Flatpak versions for (Blender, LibreOffice, VLC). None of them show multiple versions; I can still only see the PackageKit backend versions, even if I change the active source to Flatpak under Settings > Application Sources.
Never mind, I hadn't added Flathub. That's tracked by https://bugs.kde.org/show_bug.cgi?id=387718 *** This bug has been marked as a duplicate of bug 387718 ***