Comment 27 for bug 1929657

I should have added that it would be best in this case to have the debug mode enabled for udevd, too:
$ cat /etc/systemd/system/systemd-udevd.service.d/override.conf
[Service]
Environment=SYSTEMD_LOG_LEVEL=debug
$ sudo systemctl daemon-reload && sudo systemctl restart systemd-udevd
$ journalctl -b -u systemd-networkd -u systemd-udevd --no-pager
(instead of -b you may do '--since="today"' and/or '--until="5 minutes ago"' or so to reduce the output)

You may then trigger the udev rules directly, like:
$ sudo udevadm trigger
and once indirectly via netplan (and maybe via 'ip').