Unable to edit project on approved purchase order

Summary:

We are trying to edit the project field at Approved PO but we were unable to do it as it was greyed out, we thought that was the intended function, but we found out on other PO it was able to be edited on the same status while other again cannot and we want to know why on other PO it was able to be edited while the others unable to be edited

status open

Project field greyed out

While this PO with the same status is able to be edited

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