The problem is back.
Just observed a person who I had applied a user defined rington rang only for the first time but today it was back to default ringtone. Now this person is not in any group, all I have done is added him in the speed dial but I don't think that should cause an issue. In this case the ringtone applied was a stock Samsung ringtone which is pre installed. but still this problem exists. In the past it was ringtones I had added to the SD card.