An ipv4 OutboundBindAddress prevents evdns from using ipv6 nameservers
We try to apply the OutboundBindAddress to every nameserver -- but it won't work with IPv6 ones.
One solution would be to let eventdns/evdns take two such outboundbindaddress options.
We've had this issue for as long as we've supported IPv6 nameservers and OutboundBindAddress -- I guess maybe nobody wanted to use both at once? Hence setting the priority low.