I am running Solr built off branch_4x, and thanks to some help from IRC (firstname.lastname@example.org) we've determined that we have an incompatible index situation where we have indexes built with 4.9 that we can read but not index into further or update. In this situation, if I try to add a replica, this also fails, however, the only log ouput (at WARN threshold) is:
16:21:58.156 [RecoveryThread] WARN org.apache.solr.update.PeerSync - no frame of reference to tell if we've missed updates
...and the replica comes up green. I think this might indicate a missing integrity check on replication but certainly IMO a replica should report as green/active if it is not on the same revision as the leader, or at least if it has never been on the same revision as the leader.
Shards built with the currently running version of Solr are able to successfully ADDREPLICA as expected.