Hi Kavi
Is the PO Workflow a custom workflow?
Please check the definition of the Workflow. Maybe, the Workflow is initially going in a temporary exception (and a mail is configured / coded to be sent ....from your (custom) workflow) and later it is picked by the error handling job and executed successfully. (example of cases where the PO is locked)
Also, it would be good to see the technical WF log of that approval process.
Regards,
Modak