Well, uh, with Namecheap it should work just as fine as everywhere else. But as I said, I never used IPv6 and do not have anything on Namecheap anymore since I host my DNS server myself. Just a dumb question but are you sure you added the AAAA for the domain root correctly? Back when I used the web UI for DNS providers it was sometimes very confusing. Maybe you could test a subdomain? Like minecraft.yourdomain.com?
Also: you can query google.com if you want examples of how stuff looks when you get answers back.