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.