This article says the same thing, but no updates in a few weeks. Curious to see if there's something I'm missing in my config anywhere. It's definitely a bug in Ubuntu, as I had no issues with Pihole on a RPi3 running Armbian.
Another link supporting NetworkManager causing issues with eth0 at startup:
https://www.reddit.com/r/pihole/comments/44z1b9/how_to_make_pihole_start_automatically_when_i/czvege9/