This cannot be determined using the MTU values on ASA or simply from the log above. We need to take packet captures on the Outside nad DMZ2 interfaces of ASA in order to determine which host is not complying to the MSS values advertised in the first communication.
I'm not sure what is the IP of the DMZ host, so I'll take two, public_ip & private_ip. With these assumptions, capture commands would look like these:
access-l cpo permit ip host 18.104.22.168 host public_ip
access-l cpo permit ip host public_ip host 22.214.171.124
capture capo access-l cpo buffer 1000000 packet-length 1518 interface outside
access-l cpi permit ip host 126.96.36.199 host private_ip
access-l cpi permit ip host private_ip host 188.8.131.52
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...