I have had same problem - it persists in 3.20 but here is what I do to get them running again.
Try this script on all your machines on boot and see what happens
echo ‘Reseting Networking’
systemctl restart systemd-networkd
sleep 10
echo ‘Stopping Socket’
systemctl stop snap.lxd.daemon.unix.socket
sleep 10
echo ‘Starting Unix Socket’
systemctl start snap.lxd.daemon.unix.socket
sleep 10
echo ‘Stopping lxd’
systemctl stop snap.lxd.daemon
echo ‘Starting lxd’
systemctl restart snap.lxd.daemon
sleep 10
lxc cluster list
The whole conversation is here… is if this helps/