See:
https://lists.torproject.org/pipermail/tor-talk/2016-January/040074.html
My takeaway is that there's little point to setting an IPv6 DirPort at this point. Clients will rarely connect to it, and relays won't connect to it at all. Furthermore, "In 0.2.8, clients assume that the IPv6 DirPort is the same as the IPv4 DirPort" already. If that's still true, might as well leave the v4 port advertised, and if a v6 client comes along it will try the (non-advertised) v6 port anyway.
It seems like all of this is in flux though. Perhaps someone can provide an update on the current progress.