I’m pretty sure that you need to add a bit to your app.yml that tells nginx to recognize your ipv6 address and pass through the client address. I haven’t figured out how to do that yet myself, though. The topics about running a reverse proxy have ipv6 examples, which provide hints.
This is on my list of things to figure out, but I have a rather long list. I ended up disabling ipv6, I think because of this problem, but maybe due to some other ignorance of ipv6.
(Jeff you can get an ipv6 block routed, but you have to ask for it and then configure your hosts to use it.)