The IPv6 protocol isn't using or accessing a DNS server at this point because it isn't used to go online with...yet, although it may be in the near future. It's still used internally for certain networking functions in Windows at this point.
The preferred DNS for IPv4 would be the first one on the list of two DNS servers. The second DNS entry is a back up DNS.
Some info on IPv6 and it's use.
IPv6: The future of net addresses explained | News | TechRadar