DnyDNS not updating AAAA for my hostname

User_MDT3Y

I have been running DynDNS for a long time using what is now described as the legacy URL update mechanism, usign an update URL string that looks like: "http://[USERNAME]:[PASSWORD]@members.dyndns.org/nic/update?hostname=[DOMAIN]&myip=[IP]" 

I want to add IPv6 support by updating the AAAA record for my hostname, so I set up a second updater using the same URL, just with the IPv6 address in the IP payload. To ensure that something was updating, I first created a AAAA record manually on DYN with the wrong address, then launched the client. A difference should have been detected and an update made, but all I am getting back are responses with 'nochg 72.229.57.79' (that IP address is my IPv4 address.). Checking the authoritative service verifies that no updates to AAAA records are taking place.

So what is the secret sauce to update AAAA records? I did find a reference at https://help.dyn.com/managing-dynamic-updates/, but all it had to say on the topic was "for details on dynamic updates to IPv6 host records, please contact your Dyn Concierge."