Veth link is not added to bridge on host side. where to begin troubleshoot?

Given this config which worked flawlessly on one machine but fails on another, how to troubleshoot?

lxc-start xyz -l 9 doesn’t reveal anything either. veth pair is added to host side but not added to an existing bridge br-lan created by systemd-networkd

lxc.net.0.flags = up
lxc.net.0.link = br-lan
lxc.net.0.type = veth