A phased invoice does not show the purchase order number associated with a project phase. Why?
For Draft or Unprocessed invoices, you can enter or update the purchase order number directly on the Invoice Overview tab. This lets you specify the appropriate purchase order number for the invoice without changing the purchase order number on the project.
For Finalized invoices, the purchase order number is display-only and cannot be edited directly on the invoice.
The existing project-level behavior may still apply when the invoice is initially created. For phased invoices, verify the current behavior for whether the parent project purchase order number or the individual phase purchase order number is populated automatically before retaining the previous statement about the Joint Invoice template. You can check out this FAQ: How do I show PO number on invoices.