[network-manager] Explicitly configuring an eth interface with netplan makes ipv4.method=manual

If we have these two files in /etc/netplan:

$ cat /etc/netplan/00-default-nm-renderer.yaml
network:
  renderer: NetworkManager

$ cat /etc/netplan/00-snapd-config.yaml
network:
  version: 2
  ethernets:
    eth0:
      dhcp4: true

Then

$ nmcli connection show eth0
...
ipv4.method: manual
...

It turns out that the "dhcp4: true" line is not respected and we end up in a system where the eth0 connection has method "manual" for setting the IP address. We end up with a badly configured system where we be without DNS addresses in /etc/resolv.conf, for instance.

Other strange things happens, for instance:

$ nmcli c modify eth0 ipv4.method auto

creates a new connection with the same name (eth0), instead of modifying the previous one.