Reopened PO is not automatically closing once fully invoiced

Summary:

A PO had one line which was fully invoiced so the PO was automatically closed (header and line 1 status was Closed). The buyer reopened that PO which changed the PO header and line 1 status to Open. The buyer added two lines to the PO. Those two lines were fully invoiced and closed. However, the PO header and line 1 status remains as Open. Is this expected behavior or should this PO be automatically closing because all lines have been fully invoiced?

Content (required):

Since the PO line 1 was previously fully invoiced and closed, we are thinking that there is no systematic trigger for line 1 to close again. Should the system be automatically closing this PO? Or does the buyer have to manually close the PO?