This is by design. If you have autonumbering enabled, the PO number will only be displayed after you save the PO form.
This was changed because of requests by users of Track-It! 11 and prior versions as far back as I can remember. The complaint in those versions was that if a PO was added and then cancelled before saving, the number was already incremented so if you clicked add and cancel 5 times, you would waste 5 PO numbers. The new version of Track-It! was changed to stop that from happening and now will only commit a PO number when the user clicks Save.