Page History

...

In 3., the leader will wait for its current epoch to commit by waiting for the LeaderChangeMessage to commit. This is required to guarantee that the two competing voters sets, the one from a previous leader and the one from the current leader, only differ by at most one voter. Waiting for the current epoch to commit means that there cannot be some other competing voters set from another leader that can later override this leader's new voters set. See bug in single-server membership changes for more details on this.

In 47., the new replica will be part of the quorum so the leader will start sending BeginQuorumEpoch requests to this replica. It is possible that the new replica has not yet replicated and applied this VotersRecord so it doesn't know that it is a voter for this topic partition. The new replica will accept the BeginQuorumEpoch RPC even if it is doesn't believe it is a member of the voters set. 

...