When you add a normal replica set member, while it's in initial sync it still counts as a voting member towards the majority commit point, but its data is inconsistent, it cannot serve reads, and it cannot become primary. This can lead to a case where a majority of the voting nodes of the set are online but no primary can be elected.

The recommended procedure for adding a new node to a replica set should be to add it with priority:0 and votes:0, and then once it has finished initial sync and transitioned into SECONDARY state do another reconfig to make it voting and electable.