I had some similar looking problems some releases back. Using a separate carp if for ipv6 mostly fixed it. Didn't write down the exact problem, though.
- IPv6 and carp(4) problems Stefan Rinkes
- Re: IPv6 and carp(4) problems Jussi Peltola
- Re: IPv6 and carp(4) problems David Newman
- Re: IPv6 and carp(4) problems Stefan Rinkes