I am guessing you have configured both split-tunneling and split-dns. I am guessing the problem is with the split-dns. Try to disable split-dns and see if the problem goes away. If the problem goes away, then look at your split-dns config.
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...