Karjala's blog

When the env var QT_QPA_PLATFORM is set to wayland-egl (as used to be suggested by this page), the KDE Caps Lock indicator icon in the systray would not appear, and the system settings window would not show the Virtual Desktops screen, as well as a few more systemsettings pages would not appear. Also the clipboard systray app of KDE would misbehave.

Changing its value to just wayland (as suggested by the above link now) fixes these problems.