IPv6-only client fails to bootstrap on microdescriptor fetch/fallback

For a fresh client (nothing cached) when configured with "ClientUseIPv4 0" + "ClientUseIPv6 1" (and client is also on a network with no external IPv4 address).

Hangs at 45%, Asking for relay descriptors. Running on 0.2.9.7-rc.

Works fine with UseMicrodescriptors 0. ("ClientPreferIPv6DirPort 1" + "ClientPreferIPv6ORPort 1" have no affect.)

Attaching torrc and a log @ loglevel info.

CCing teor here since I vaguely remember having a conversation with him at last tor meeting where I believe he mentioned to try using "UseMicrodescriptors 0"; which (now that I've had a chance to test) seems like a good workaround at the moment.

Can't seem to find another ticket for this exactly, and just wanted something to refer to (and post some logs and info on), so please excuse if it’s actually a dupe. Possibly related: #20916 (moved).

On the mobile side, Apple’s starting to require that apps work in IPv6-only networks. And I’ve noticed that I’ve recently been assigned IPv6/NAT64 (with no external IPv4 address) on T-Mobile lately. But of course this also affects non-mobile IPv6-only networks.

(On the other hand, I'm quite happy that this all does work under IPv6 with the "UseMicrodescriptors 0" workaround.)