Use systemctl reload snap.lxd.daemon as snap restart lxd or systemctl restart snap.lxd.daemon will cause a “clean” shutdown of the LXD daemon, shutting down all the containers in the process.
When the snap package updates (refresh), a reload is performed, specifically so that containers don’t get respawned. Unfortunately we can’t make snap restart lxd do that as what it does is call systemctl restart lxd which in turn is the same as systemctl stop and systemctl start and we do need systemctl stop to shutdown all containers as otherwise we’d get data corruption when you shutdown or reboot the server.