When syncing from 4.4, the exhaust cursors update lastKnownCommittedOpTime after sending out a batch so that it will wait for new data in the next round of getMore. However, we don't have that on 4.2 branch. So lastKnownCommittedOpTime stays stale and because of commit point propagation, a 4.2 sync source keeps sending out empty batches to 4.4 syncing nodes.