MongoDB unrecoverable replication error
let me share you my understanding on this.

question 1:****why is MongoDB trying rollback in the first place
When there is a node switch, documents which are not yet written on secondaries needs to be rollback. As M1 was primary and at the time of switching some documents were not sync they must be in your oplog but as your server was restarted in the mean time M2 become primary and rest of replica members starts syncing with new primary. Now your M1 came up and found that his last operation was not written on other members so it needs to be rollback.

Question 2:****where is operation with timestamp of Jul 28 07:45:12:2 is coming from
check this timestamp in oplog.rs you will find your answer as this would be the same document(rollback)