You could try adding one of the "log packets" actions for the specific signature. It may or may not have what you're looking for though. If it doesn't you're kind of SOL unless you can magically get a trace at the right time.
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...