rejecting sales item doesn't update document flow

I create a return document with reference to a sales order. Document flow is updated in table vbfa. vbfa-RFMNG keeps the reference quantity. When I change quantity in the return order, frmng is updated automatically. But when I set reject reason to the return order, frmng is not cleared. Is it a bug? I tried create order with reference to contract and do the same thing the frmng is cleared. How to make return to behave same way?