I keep getting these orphaned ipv6 routes, or rather Veth devices that are not being deleted I guess, this was not a one time thing. Any idea how to figure out what is causing this?
Try manually removing the route as before and then enabling debug mode:
sudo snap set lxd daemon.debug=true
sudo systemctl reload snap.lxd.daemon
Then try starting and stopping the affected container and see if there are any logs in /var/snap/lxd/common/lxd/logs/lxd.log that show a failure to remove the NIC device on stop.