IPv6 Reachability Test Fails on FreeBSD and CenturyLink IPv6

When I run a dual-stack Tor relay on FreeBSD, and use certain ISPs like CenturyLink, I get a warning with the IPv6 reachability test:

Aug 26 13:18:29.000 [notice] Now checking whether IPv6 ORPort [REDACTED]:143 is reachable... (this may take up to 20 minutes -- look for log messages indicating success)
Aug 26 13:21:31.000 [notice] Self-testing indicates your ORPort 1REDACTED:143 is reachable from the outside. Excellent.
Aug 26 13:23:32.000 [notice] Performing bandwidth self-test...done.
Aug 26 13:36:29.000 [notice] Auto-discovered IPv6 address [REDACTED]:143 has not been found reachable. However, IPv4 address is reachable. Publishing server descriptor without IPv6 address.

I am able to reach the IPv6 address and port from a VPS.

CenturyLink uses 6rd for IPv6. I could use Comcast, but it's not an option for me (despite being available) since they don't offer symmetrical speeds and cost a lot more, especially since CL gives me fiber and not slow DSL which is good for relays :-).

Google Fiber/Webpass does not have this issue (despite the same torrc) since Webpass has native v6, but they aren't in my new address.