Note that the *proper* way to place eth0 into a bridge while also being under systemd-networkd management is to create a .netdev for the bridge and assign eth0 to it, e.g.:
Or alternately, just remove the .network file for 'eth0' so it's not under systemd-networkd control, and manually and/or externally control it (i.e. manually bring up eth0 and manually place it into br0).
Note that the *proper* way to place eth0 into a bridge while also being under systemd-networkd management is to create a .netdev for the bridge and assign eth0 to it, e.g.:
$ cat 10-br0.netdev
[NetDev]
Name=br0
Kind=bridge
cat 10-netplan- eth0.network
[Match]
Name=eth0
[Network] sing=ipv6
DHCP=ipv4
LinkLocalAddres
Bridge=br0
[DHCP]
RouteMetric=100
UseMTU=true
Or alternately, just remove the .network file for 'eth0' so it's not under systemd-networkd control, and manually and/or externally control it (i.e. manually bring up eth0 and manually place it into br0).