0.4.5 relay with autodetected unreachable ipv6 port says it's publishing but then doesn't

I restarted an old relay on Tor 0.4.5, and to my surprise, it auto detects some sort of ipv6 address, even though I have an "Address" line listing an IPv4 address explicitly.

Feb 06 03:17:57.448 [notice] Now checking whether IPv4 ORPort 128.31.0.39:9005 is reachable... (this may take up to 20 minutes -- look for log messages indicating success)
Feb 06 03:17:57.457 [notice] Now checking whether IPv6 ORPort [2603:400a:ffff:bb8:42a8:f0ff:fe75:6090]:9005 is reachable... (this may take up to 20 minutes -- look for log messages indicating success)
Feb 06 03:17:58.264 [notice] Self-testing indicates your ORPort 128.31.0.39:9005 is reachable from the outside. Excellent.
Feb 06 03:19:00.478 [notice] Performing bandwidth self-test...done.

and then it doesn't find the IPv6 address reachable (not surprising), and it sits there. I was preparing to file a "woah, are we going to lose a lot of relays?" ticket, when 19 minutes later, I get this line:

Feb 06 03:37:55.776 [notice] Auto-discovered IPv6 address [2603:400a:ffff:bb8:42a8:f0ff:fe75:6090]:9005 has not been found reachable. However, IPv4 address is reachable. Publishing server descriptor without IPv6 address.

Well great!

But then it doesn't actually publish any descriptor.