I assume you use 7.x.x softver on the PIX or ASA. If the situation is it, the skinny inspection drops the ACS replication packets. Unfortunately there is a bug that you can't see the packets that are dropped by the SKINNY inspection.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...