Forum topic: https://airvpn.org/topic/25140-the-issue-your-browser-is-avoiding-ipv6/ Ipv6 test: http://test-ipv6.com/
In the forum thread AirVPN writes:
Our assigned ULAs are in fde6:7a:7d20::/48 which is inside the range officially reserved to ULA so we don't understand why a browser should discriminate against them in favor of a local IPv4 address...
I hope to answer this question in this document.