SUMMARY
Frequently Discover does not find updates from Arch Linux repos. Then I run 'pacman -Syu' and all updates are installed as expected. When Discover does not find updates from Arch repos available I see this output in Konsole:

$ plasma-discover
adding empty sources model QStandardItemModel(0x55a584a5d7b0)
org.kde.plasma.libdiscover: Couldn't find a category for  "fwupd-backend"
file:///usr/lib/qt/qml/org/kde/kirigami.2/private/PrivateActionToolButton.qml:74:5: QML Binding: Binding loop detected for property "value"
kf.sonnet.core: Sonnet: Unable to load plugin "/usr/lib/qt/plugins/kf5/sonnet/sonnet_hspell.so" Error: "Cannot load library /usr/lib/qt/plugins/kf5/sonnet/sonnet_hspell.so: (libhspell.so.0: cannot open shared object file: No such file or directory)"
kf.sonnet.core: Sonnet: Unable to load plugin "/usr/lib/qt/plugins/kf5/sonnet/sonnet_hspell.so" Error: "Cannot load library /usr/lib/qt/plugins/kf5/sonnet/sonnet_hspell.so: (libhspell.so.0: cannot open shared object file: No such file or directory)"


Then I run the following command, reboot, and the problem is solved until it occurs again apparently without any reason:

$ sudo rm -R /var/lib/PackageKit/alpm

Operating System: Arch Linux
KDE Plasma Version: 5.24.1
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.2
Graphics Platform: Wayland