Change Order getting rejected automatically

Summary:

We are trying to test a scenario wherein a Supplier initiates the Change Order using supplier portal and Buyer rejects/accepts the change Order & revise Purchase Order again. When the buyer takes an action on the Supplier initiated change order (accept/reject) and edit the Purchase Order, a new Change Order is created cancelling the previous Supplier initiated Change Order.

The Buyer then Submits the Change Order but the PO gets rejected automatically after sometime.

Please guide us in understanding the reason behind the PO getting rejected automatically.

Is this an expected behavior ? if not, please help us in understanding why the PO got rejected