With systemd-networkd on the 20.04 server, the /64 prefix does not bounce between 0s and 1s, but stays at 0s, but is not removed (nor is the /128).
With systemd-networkd on the 20.04 server, the /64 prefix does not bounce between 0s and 1s, but stays at 0s, but is not removed (nor is the /128).