There is a race where the sync source's most recent oplog entries could become visible and be read by downstream nodes before the sync source updates its heartbeat map with its new last applied OpTime. This can cause downstream nodes to get stale lastAppliedOpTimes in metadata which can be a problem for OplogFetcher::checkRemoteOplogStart.
This will only cause the OplogFetcher to return early and choose a new sync source, so it should not cause harm beyond unnecessary sync source changes and some very quick initial sync restarts.
We should remove the check that the remote last applied OpTime is greater than or equal to the local last fetched OpTime in OplogFetcher::checkRemoteOplogStart when "requireFresherSyncSource" is false. This will also require changing the comments that explain the boolean's meaning.
An alternative is to use the max of the metadata lastOpApplied and the last OpTime in the batch as the remote last applied OpTime in OplogFetcher::checkRemoteOplogStart.