Comment 0 for bug 1817655

NetworkManager 1.15.2 or later (as included first in Ubuntu 19.04 Disco Dingo) now requires that files in /{etc,run}/NetworkManager/system-connections end in the suffix '.nmconnection' as per the following commit:
https://github.com/NetworkManager/NetworkManager/commit/648c256b9014198aac388097e410999c68c4b452

NetPlan writes files as /run/NetworkManager/system-connections/{name} without any suffix. On Ubuntu 19.04 these files are ignored and the configuration is not applied.

If you rename the files to end in '.nmconnection' then you can see the interface configuration loaded by NetworkManager.

root@optane:/run/NetworkManager/system-connections# ls
 netplan-eno1 virbr0.nmconnection 'Wired connection 1.nmconnection' 'Wired connection 2.nmconnection'

root@optane:/run/NetworkManager/system-connections# cat /etc/netplan/03-netcfg.yaml
# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    eno1:
      addresses: [10.48.134.22/25]
      gateway4: 10.48.134.1
      nameservers:
        addresses: [10.48.134.6]