Comment 6 for bug 2040153

I can confirm this fixes the issue for me, too, using network-manager 1.44.2-1ubuntu1.2 from mantic-proposed.

$ sudo apt install -t mantic-proposed network-manager
[...]
$ LC_ALL=C apt list -i network-manager
Listing... Done
network-manager/mantic-proposed,now 1.44.2-1ubuntu1.2 amd64 [installed]

$ nmcli con add type dummy connection.interface-name dummy0
$ sudo cat /etc/netplan/90-NM-4189c63a-ecee-4a39-90f3-fffad2b96d0b.yaml
network:
  version: 2
  dummy-devices:
    NM-4189c63a-ecee-4a39-90f3-fffad2b96d0b:
      renderer: NetworkManager
      networkmanager:
        uuid: "4189c63a-ecee-4a39-90f3-fffad2b96d0b"
        name: "dummy-dummy0"
        passthrough:
          connection.interface-name: "dummy0"
          dummy._: ""
          ipv4.method: "disabled"
          ipv6.addr-gen-mode: "default"
          ipv6.method: "disabled"
          ipv6.ip6-privacy: "-1"
          proxy._: ""
$ nmcli d show dummy0
GENERAL.DEVICE: dummy0
GENERAL.TYPE: dummy
GENERAL.HWADDR: 32:89:DC:E0:74:2F
GENERAL.MTU: 1500
GENERAL.STATE: 100 (verbunden)
GENERAL.CONNECTION: dummy-dummy0
GENERAL.CON-PATH: /org/freedesktop/NetworkManager/ActiveConnection/10
IP4.GATEWAY: --
IP6.GATEWAY: --

$ nmcli con del dummy-dummy0
$ LC_ALL=C nmcli d show dummy0
Error: Device 'dummy0' not found.
$ sudo LC_ALL=C cat /etc/netplan/90-NM-4189c63a-ecee-4a39-90f3-fffad2b96d0b.yaml
cat: /etc/netplan/90-NM-4189c63a-ecee-4a39-90f3-fffad2b96d0b.yaml: No such file or directory
$ journalctl -u NetworkManager -e # I cannot spot any errors in here