Currently, we've implemented failover to ipv4 scenery , it's supported by ISC DHCP that it's base on RFC https://datatracker.ietf.org/doc/html/draft-ietf-dhc-failover . however , for IPv6 hasn't been contemplated, as a solution is provided by ISC DHCP is Kea High Availability :
https://kb.isc.org/docs/aa-01617
https://gitlab.isc.org/isc-projects/kea/-/wikis/designs/High-Availability-Design
basically is HA to dhcpv4/v6 , below described the official documentation :