Hi Gangadhar,
I think this is a program error. I am experiencing the same issue and I am getting the same result as you. I have just created an OSS message for this.
For your reference, the config node is "Maintain Field Mapping" under common functions > DFM > attrib sync. Main and sub grouping PPO, scenario Initial Create, source and target field SHORT_TEXT. If all of these settings are in place for you and it still does not work, then I think it's an SAP error in your system as well.
I would be interested to know if it works in your system and your current SP level. I am on PPM5.0 SP11.
Thanks,
Lashan