The bigger issue is that neither DBUS_SESSION_BUS nor DISPLAY is forwarded through pkexec which prevents KUniqueApplications (and their KF5 counterpart one presumes) from starting as uniqueness is implemented through dbus and auto-starting a session dbus at least on X11 requires DISPLAY to be set (not that I know why).
The bigger issue is that neither DBUS_SESSION_BUS nor DISPLAY is forwarded through pkexec which prevents KUniqueApplications (and their KF5 counterpart one presumes) from starting as uniqueness is implemented through dbus and auto-starting a session dbus at least on X11 requires DISPLAY to be set (not that I know why).