For summary, This is a dbus-glib upstream issue (https://gitlab.gnome.org/GNOME/glib/issues/1831). An ongoing merge request exists at https://gitlab.gnome.org/GNOME/glib/merge_requests/1176.
It's most likely a race condition on auth which makes a gdbus server to receive invalid client UID/GID. It breaks the typical same-user authentication check. Original bug was from ibus. Because of this, Qt 5.x apps failed to connect to ibus with a proper security fix (#940267).
