From my experience, either in ATO or MTO this kind of verification is really lacking in SD/PP. We had a somewhat similar issue when users were making changes in sales orders even after the PO was already released to manufacturing. They could even apply reason for rejection. We ended up adding some ABAP code in USEREXIT_CHECK_VBAK (may need to use a different one, based on the specific requirement) to check PO status and throw an error message.
Maybe something could be done using custom statuses, but we found their use to be rather limited.
Hope this helps.
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