In SERVER-84195, we changed sync source selection to use lastWritten instead of lastApplied. However, when we do oplog fetching , we use a local read against the sync source, which will be transformed to a PIT read at lastApplied if the sync source is a secondary. This can cause a problem in the following case(T1 < T2 < T3):

• Node A is a secondary and lastWritten(A)=T3, lastApplied(A)=T1
• Node B is another secondary and lastWritten(B)=T2
• Node B changes its sync source to be A because A's lastWritten is greater than B's.
• Node B sends a find cmd to Node A but Node A turns that into a snapshot read at T1, which will give Node B nothing. 
• Node B goes into an unexpected rollback.