Neutron HA backup instances have the same IP/MAC addresses as the master instance and they have IPv6 forwarding enabled by default. This causes them to subscribe to different multicast groups and they may respond to queries coming from the external network. This traffic will make the upstream switch learn the MAC address on a different port, disrupting existing traffic to the master instance.
Disable IPv6 forwarding on backup instances and restore it on failover. Traffic will not leave the backup instance to the upstream switch and will not disrupting existing connections with the master instance.