Dear Aravind,
Is PMEX your defined custom control key ? In standard I see PM02.
Check if the Indicator: Scheduling external op. with standard values is set,
As per SAP help, "If the indicator is not set then these objects are scheduled using their planned delivery time when they are externally processed.".
It seems to me in your case this indicator is checked in your control key.
Uncheck this indicator and test and let us know.
Best Regards,
Ameya B.