Benoire:

 

IPv6 is broken if you have more than one network defined in your controller; assigning different prefix IDs causes the USG to fail provisioning and eventually break and assigning the same prefix id (or leaving it blank) causes it to not assign an IPv6 to the LAN port.  I have only assigned it to my main lan so its working but my wLAN which runs of the 2nd usg port is not able to get one due to the above.  Its being well documented by others on the official forums with a large error code.

 

Oh right - does this still happen on 5.9.x? I've never run into this issue (and have IPv6 enabled on a site with a bunch of VLAN's on an USG Pro). In your case why are you serving your UniFi off the 2nd USG port instead of using a switch?