Merlin, don't know if you can test with ipv6, but if you can, set "Connect to DNS Server automatically" to "Disable" on the ipv6 page, and try using some public ipv6 DNS servers, like the OpenDNS ones:
http://www.opendns.com/about/innovations/ipv6/
The only access I have to IPv6 is a 6in4 tunnel unfortunately.
Asus did quite a number of changes to IPv6 in recent firmwares, so it's possible that either one of these changes broke it (if you have an AC68 or AC56, try the original firmware and see if it has the same issue), or an error slipped by when I merged their code in. Testing with the original 374.542 or 374.583 FW would help determining that.