Condition in "Manage purchasing document approval" is not work when cancel PO

My case is that i want to "Cancel" the PO and then submit for approval base on PO document amount. But i find below issue,

Document line total amount or document ordered amount Condition in "Manage purchasing document approval" is not work when cancel PO only without any change in other terms.
But The amount limit condition cannot be meet.

If using Document line ordered amount, the condition cannot be meet if multiple lines in PO.

Would anyone can help to explain why ?