Hi Murthy,
The functionality that you are talking about is a standard behavior.
As rightly pointed out by Ivor, as there is no new non-statistical posting being created, during the SD release, so the orginal payment posting date is shown in the SD released document.
From a finance prepective, it should be as it is, other wise, it would cause issue in the month-end reconciliation, as well as in the next invoice generation process.
If you want to capture the actual released date, then there are other ways to find that out, but I wouldn't recommend you to change the posting date of the released document.
Hope it helps
Thanks,
Amlan