I can confirm this bug. Very annoying.
Without this packages, the KDE session and all user units had their own dbus server/bus. Only after installing the dbus-user-session package all share the same bus under /run/user/1000/bus.
However, from examining /etc/X11/Xsession.d/*, in particular the call to dbus-update-activation-environment --systemd, that suggests that this should be the default configration.
Thanks Mirosław for your research!
