If a voting member goes through initial sync, it will not be able to vote for the commit quorum until it finishes the collection cloning phase and start applying the index build oplog entries. This can block the index build from committing on other nodes. The primary node should be able to tell if a node is going through initial sync (as documented here). We should adjust the commit quorum of the index builds automatically when the primary finds out a secondary is going through an initial sync. The newly started index builds should set the commit quorum without including the syncing node, too.