Quickly changing netplan configuration sometimes causes systemd-networkd to fail

MAAS 2.7 adds support for testing network configuration. The network configuration is only applied when a connectivity test is being run, otherwise the default network configuration(DHCP) is used. If multiple connectivity tests are being run the configuration is reapplied each time.

For example if the smartctl-validate, internet-connectivity, gateway-connectivity, and rack-controller connectivity tests are selected the following will happen

1. Machine boots into the MAAS ephemeral environment
2. Networking is automatically configured to DHCP on the boot device as the default
3. smartctl-validate test runs
4. netplan apply custom networking
5. internet-connectivity test runs
6. netplan apply default networking
7. netplan apply custom networking
8. gateway-connectivity test runs
9. netplan apply custom networkingd
10. netplan apply custom networking
11. rack-controller-connectivity test runs
12. netplan apply custom networking

When one of the connectivity tests runs quickly enough it causes systemd-networkd to fail with

systemd-networkd.service: Start request repeated too quickly.
systemd-networkd.service: Failed with result 'start-limit-hit'.
systemd-networkd.service: Failed to start Network Service.

This results in the system having no networking which causes all remaining tests to timeout and the machine goes into a failed testing state. If I access the machine through the console and run systemctl restart systemd-networkd networking comes back up.