Quantityordered in purchase requisition isn't update when the po delete.

Dear Experts,

     I create a pr and create a po reference the pr. I think if the po has delete, the quantity orderd in pr must be upate to 0 and the pr must be open

again. But the system don't act like this, as the follow picture, the quantity orderd isn't upate even though the po has delete. Not all the pr like

this, I only find one pr has the problem. I don't know how to resolve this. I need some help, thanks very much!