We're experiencing delay on CARP failovers with IPv6 in multiple setups at the moment.
Transition from BACKUP to master works fine and Gratuitous ARP is sent for all IPv4 addresses immediately. The unsolicited Neighbor Advertisement for IPv6 seems to be missing (at least we don't see it when capturing traffic with TCP-dump).
Future Neighbor Solicitations will be answered by the current CARP MASTER correctly but until then traffic is still arriving at the BACKUP (former MASTER).
Google returned this blogpost: https://www.stoklasa.one/carp-with-ipv6-protocol/
Josef Stoklasa seems to think that this is normal behaviour but looking at carp_master_down_locked function here looks like that there should be an unsolicited Neighbor Advertisement: