@stgraber If I remove ConditionPathExists=/dev/virtio-ports/org.linuxcontainers.lxd from the /lib/systemd/system/lxd-agent-9p.service then the agent starts OK on boot.
So something is delaying that file from being present and the lxd-agent-9p service unit is starting early enough to miss it. But apparently this doesn’t affect Ubuntu VMs.