Cannot edit an open Purchase order line?

Dear Experts,

I tried to edit the quantity of an open Purchase order line but cannot.

Could you please help me if you have ever met this issue? The scenario is as below:

I confirmed a PO with ordered quantity line of 50 pcs.

I post a receipt and invoice for 20 pcs. The remaining quantity is 30 pcs. PO’s status is open.

However when I click button Request Change (because the PO uses approval workflow) then Edit, the PO approval status changes to “Draft”, but the PO line is still frozen.

I just want to edit the ordered quantity to 40 pcs but recognized that I cannot edit any information on the PO line, as well as the button Product receipt is unavailable.

Any of your recommendation will be highly appreciated. Thank you in advance for your support!