It is not possible to connect to dbus bus running on host from within a user-namespace, a typical containers setup.
The "EXTERNAL authentication" mechansim fails to verify the UID credential passing via the message against the out-of-band credential, due mismatch in user-id crossing user-namespace.

Frameworks like sd-bus, gdbus has already switched to sending empty value instead of UID in DATA payload to fix the issue.
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2832

systemd/systemd@1ed4723