Ignore NAs when they are received for BACKUP CARP address, so ND6 cache will not changed.
Also do not send NS from BACKUP CARP addresses, this won't confuse network switch and it won't complain about MAC addresses flapping.
OpenBSD has similar [[ https://github.com/openbsd/src/blob/41886b6c0e78ae59c3eabe22ce0ffad15773ac4f/sys/netinet6/nd6_nbr.c#L672 | solution ]] for NAs.