Comment 5 for bug 1979277

I can now reproduce the issue inside a classic Jammy LXD container, using the network-manager snap from 22/stable.

When appending the reproducer script with this:
netplan get
ls -la /run/NetworkManager/system-connections
ls -la /run/systemd/network

I can see that netplan and sd-network are doing the correct things. All configuration are as they should be according to the default renderer displayed by "netplan get" and sd-network picks-up the connection when it is asked to and keeps it as "unmanaged" when NetworkManager is supposed to take control.

The NetworkManager deb also seems to work correctly. But the NM snap seems to cause this problem as can be seen from "journalctl -u snap.network-manager.networkmanager -e" logs. Most probably related to this:
https://git.launchpad.net/~snappy-hwe-team/snappy-hwe-snaps/+git/network-manager/tree/snap-common/bin/snap-config.sh?h=snap-22#n150

When calling netplan's "Apply()" dbus method, the NetworkManager systemd service is re-started. on restart the network-manager snap installs itself as default renderer and calls the "Apply()" method again, recursively. This doesn't look right.

IMO the problem is that calling "systemctl restart snap.network-manager.networkmanager" will reset the default renderer back to NetworkManager, as can be seen in the following log:
root@jj:~# ./repro-lp1979277.sh
+ netplan_cfg_p=/etc/netplan/00-default-nm-renderer.yaml
+ '[' -f /etc/netplan/00-default-nm-renderer.yaml ']'
+ cat
+ dbus-send --system --type=method_call --print-reply --dest=io.netplan.Netplan /io/netplan/Netplan io.netplan.Netplan.Apply
method return time=1656424157.108626 sender=:1.20 -> destination=:1.884 serial=252 reply_serial=2
   boolean true
+ nmcli d
DEVICE TYPE STATE CONNECTION
eth0 ethernet connected netplan-eth0
erspan0 erspan unmanaged --
gre0 iptunnel unmanaged --
gretap0 iptunnel unmanaged --
ip6gre0 iptunnel unmanaged --
ip6tnl0 iptunnel unmanaged --
sit0 iptunnel unmanaged --
tunl0 iptunnel unmanaged --
lo loopback unmanaged --
ip_vti0 vti unmanaged --
+ networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 gre0 ipgre off unmanaged
  3 gretap0 ether off unmanaged
  4 erspan0 ether off unmanaged
  5 ip6tnl0 tunnel6 off unmanaged
  6 ip6gre0 ip6gre off unmanaged
  7 tunl0 tunnel off unmanaged
  8 sit0 sit off unmanaged
  9 ip_vti0 tunnel off unmanaged
 10 ip6_vti0 tunnel6 off unmanaged
188 eth0 ether carrier unmanaged

11 links listed.
+ netplan get
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    eth0:
      dhcp4: true
+ ls -la /run/NetworkManager/system-connections
total 4
drwx------ 2 root root 60 Jun 28 13:49 .
drwxr-xr-x 6 root root 160 Jun 28 13:30 ..
-rw------- 1 root root 131 Jun 28 13:49 netplan-eth0.nmconnection
+ ls -la /run/systemd/network
total 0
drwxr-xr-x 2 root root 40 Jun 28 13:49 .
drwxr-xr-x 25 root root 600 Jun 28 13:49 ..
root@jj:~# ./repro-lp1979277.sh
+ netplan_cfg_p=/etc/netplan/00-default-nm-renderer.yaml
+ '[' -f /etc/netplan/00-default-nm-renderer.yaml ']'
+ rm /etc/netplan/00-default-nm-renderer.yaml
+ dbus-send --system --type=method_call --print-reply --dest=io.netplan.Netplan /io/netplan/Netplan io.netplan.Netplan.Apply
method return time=1656424160.737685 sender=:1.20 -> destination=:1.890 serial=254 reply_serial=2
   boolean true
+ nmcli d
DEVICE TYPE STATE CONNECTION
eth0 ethernet connected netplan-eth0
erspan0 erspan unmanaged --
gre0 iptunnel unmanaged --
gretap0 iptunnel unmanaged --
ip6gre0 iptunnel unmanaged --
ip6tnl0 iptunnel unmanaged --
sit0 iptunnel unmanaged --
tunl0 iptunnel unmanaged --
lo loopback unmanaged --
ip_vti0 vti unmanaged --
+ networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 gre0 ipgre off unmanaged
  3 gretap0 ether off unmanaged
  4 erspan0 ether off unmanaged
  5 ip6tnl0 tunnel6 off unmanaged
  6 ip6gre0 ip6gre off unmanaged
  7 tunl0 tunnel off unmanaged
  8 sit0 sit off unmanaged
  9 ip_vti0 tunnel off unmanaged
 10 ip6_vti0 tunnel6 off unmanaged
188 eth0 ether routable configuring

11 links listed.
+ netplan get
network:
  version: 2
  ethernets:
    eth0:
      dhcp4: true
+ ls -la /run/NetworkManager/system-connections
total 0
drwx------ 2 root root 40 Jun 28 13:49 .
drwxr-xr-x 6 root root 160 Jun 28 13:30 ..
+ ls -la /run/systemd/network
total 4
drwxr-xr-x 2 root root 60 Jun 28 13:49 .
drwxr-xr-x 25 root root 600 Jun 28 13:49 ..
-rw-r--r-- 1 root root 100 Jun 28 13:49 10-netplan-eth0.network
root@jj:~# netplan get
network:
  version: 2
  ethernets:
    eth0:
      dhcp4: true
root@jj:~# systemctl restart snap.network-manager.networkmanager
root@jj:~# netplan get
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    eth0:
      dhcp4: true

Please re-open if you feel like this is a netplan issue anyways.