DC discovery fails unexpectedly when the DNS reply contains close to a dozen of the requested _ldap._tcp.dc._msdcs.<domain>. records in the Answer section of the DNS reply packet.
Please see the attached network capture.
Work Around
If the given domain spans multiple sites, one might be able to get
around this problem by specifying the 'ads_site' property assuming the specified site consists of just one or two domain controller(s).