We suspect that checkQuorumForConfigChange on the reconfig'ing node is communicating the old config to the newly added node. Specifically, the node running the reconfig command issues a heartbeat to every member of the new config. This causes the newly added node to issue a heartbeat back to the reconfig'ing node before that node has finished processing the reconfig. The response to the new node contains the old config, which does not contain the new node, so it goes into REMOVED. It then receives a heartbeat from the reconfig'ing node when the reconfig is complete, and this time the new node gets the new configuration and joins the set.