Comment 6 for bug 1676547

Info I've gathered about this issue so far:

The netplan should be written to this location:

/etc/netplan/01-network-manager-all.yaml

And should contain:

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager

See this reference:

http://bazaar.launchpad.net/~ubuntu-core-dev/livecd-rootfs/trunk/view/head:/live-build/auto/build#L196

If the file is missing, it can be manually added.

Ubuntu transitioned to netplan right before version 1.2.2-0ubuntu8. See here:

https://lists.ubuntu.com/archives/ubuntu-devel/2016-July/039464.html

Additionally, this is related to these 2 bugs:

https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1638842
https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1658921

When 1.2.2 was released, a globally managed file was created (/etc/NetworkManager/conf.d/10-globally-managed-devices.conf), and a canceling file was also created (/etc/NetworkManager/conf.d/10-globally-managed-devices.conf).

https://git.launchpad.net/network-manager/tree/debian/network-manager.postinst#n48

For some reason, either the override file or the netplan file is missing on upgrades. Between the two of them, users are left without fully managed Ethernet and networking when upgrading from 16.04.