Comment 2 for bug 1943120

This is a known issue and has been the case for a long time: http://manpages.ubuntu.com/manpages/focal/man8/netplan-apply.8.html#known%20issues

Would the workaround described in the manpage (i.e. calling "ip link delete dev br54" manually) work for you?

I'll check what I can do about it inside the netplan CLI & DBus API.

I can confirm this happens using normal netplan set/netplan apply on an current Ubuntu Impish installation, using netplan 0.103:

root@ii:~# ip l | grep br54
root@ii:~# netplan get
network:
  ethernets:
    eth0:
      dhcp4: true
  version: 2
root@ii:~# netplan set network.bridges.br54.dhcp4=true
root@ii:~# netplan apply
root@ii:~# netplan get
network:
  bridges:
    br54:
      dhcp4: true
  ethernets:
    eth0:
      dhcp4: true
  version: 2
root@ii:~# ip l | grep br54
4: br54: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
root@ii:~# netplan set network.bridges.br54.dhcp4=null
root@ii:~# netplan apply
root@ii:~# netplan get
network:
  ethernets:
    eth0:
      dhcp4: true
  version: 2
root@ii:~# ip l | grep br54
4: br54: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000