Further info: successive restarts of systemd-networkd or netplan apply (which I've since learned simply generates files and restarts networkd) applies the rule, following by the next immediate invocation removing the rule, and so on.
Further info: successive restarts of systemd-networkd or netplan apply (which I've since learned simply generates files and restarts networkd) applies the rule, following by the next immediate invocation removing the rule, and so on.