Excess Receipt quantity

Hello,

I do have the following queries in I-Procurement :

1) Created a Requisition with quantity as 5 and received quantity 6. The system did not restrict bu just a warninh was flashed.

2) Created an AP Invoice and matched to PO with quantity as 5.

3) Invoice got validated and PO is closed.

4) In I-Procurement, modifid the receipt and returned the excess quantity i.e. 1.

According to me the receipt should not be modified once the Invoice is validated and PO is closed.

Is there a known bug in I-Procurement ?

Thanks