Charge account behaviour is working fine for requisition but not for PO

Summary:

Account segment of the requisition gets defaulted based on category code as per mapping set. which is expected behaviour as per business requirement.

Even if i change account segment of charge account whihout changeing category , account segments gets default back as per category mapping which is all good.

But for PO this is not followed, buyer can change the charge account segment without changing the category , is this expected behaviour ? Or M i missing some setup ?

Why there is a difference in the behaviour for charge account in requisition and POs ?

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