Comment 3 for bug 1770082

Are you providing network config to cloud-init? Otherwise, cloud-init is going to generate a fallback network config like your original, dhcp on one of the interfaces.

If you want to persist network changes, you need to add them to a separate netplan yaml config, or provide an updated network-config to cloud-init; otherwise it's going to regenerate the same configuration which will clobber a local change.