TJ - this isn't exactly the same issue if I understand correctly what you are doing. I ran into what I think is your issue last week. I installed using the 18.04.1 alternate server installation and then installed the 'ubuntu-desktop' meta package which pulled in network-manager. My interface was working, but not managed by network-manager until I added it as the renderer in the netplan yaml file.
TJ - this isn't exactly the same issue if I understand correctly what you are doing. I ran into what I think is your issue last week. I installed using the 18.04.1 alternate server installation and then installed the 'ubuntu-desktop' meta package which pulled in network-manager. My interface was working, but not managed by network-manager until I added it as the renderer in the netplan yaml file.
Is that the same issue you are reporting?