With RC6 replication fails in IPv6-only environments. As a result a second DC does not obtain a RID pool after the join process completed. samba-tool drs showrepl shows replication errors in this case (WERR_BADFILE IIRC).
This is due to a trivial bug introduced during the major rework of dns_ex.c in commit c54fe86a63f73543eaf9b031e146d5f647c05830, which makes reply_to_addrs not count successful lookups of QTYPE_AAAA. The corresponding code is still present in 4.0.0.
Created attachment 8409 [details]
Patch for reply_to_addrs
Created attachment 8410 [details]
Pushed to master as bdc172aca541046fd03b2b0cd69e054fe03d3a89
Created attachment 8423 [details]
Patch for v4-0-test
Andrew, can I get a review+?
Pushed to autobuild-v4-0-test.
Pushed to v4-0-test.
Closing out bug report.