If I click the 'installed' option in the bottom-left within about 1 second of opening Discover, the right panel will say "Sorry, nothing found..." If I switch to another section and back again, the list will display as expected.
Do you see the same issue if you run discover with "plasma-discover --mode installed"?
Yes, same thing happens. Here's the output, not sure if it's useful. It does start showing the installed section despite what the first line says. unknown mode "installed" file:///usr/lib/qt/qml/org/kde/kirigami.2/PageRow.qml:291: TypeError: Cannot read property 'page' of null error loading "flatpak-backend" "Cannot load library /usr/lib/qt/plugins/discover/flatpak-backend.so: (libflatpak.so.0: cannot open shared object file: No such file or directory)" QJsonObject({"IID":"org.kde.muon.AbstractResourcesBackendFactory","MetaData":{},"className":"FlatpakBackendFactory","debug":false,"version":330241}) invalid kns backend! "/etc/xdg/servicemenu.knsrc" because: "Config group not found! Check your KNS3 installation." invalid kns backend! "/etc/xdg/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation." error loading "packagekit-backend" "Cannot load library /usr/lib/qt/plugins/discover/packagekit-backend.so: (libpackagekitqt5.so.1: cannot open shared object file: No such file or directory)" QJsonObject({"IID":"org.kde.muon.AbstractResourcesBackendFactory","MetaData":{},"className":"PackageKitBackendFactory","debug":false,"version":330241}) Discarding invalid backend "servicemenu.knsrc" Discarding invalid backend "ksysguard.knsrc" falling back applications backend to -1
cannot confirm on Arch Linux, Discover 5.12.3.
Also cannot reproduce, with 5.12.3 or from git master.
It's weird that you don't seem to have libpackagekitqt5.so.1 or libflatpak.so.0 so none are loaded. I wonder why it works later.
Installing `packagekit-qt5` fixed the issue. (It also now integrates pacman repos, which I have been wondering how to fix.) Should this be listed as a dependency?
Yes, but and that would be the responsibility of your packagers (in this case, the Arch packaging team). Please file a bug for them.