Missing from where?  Vendor Maintenance, PO Tab?  Or from somewhere else? 

Yes, that's correct, vendor maintenance PO tab.

That tab only shows POs still in PO_PurchaseOrderHeader.  Check the "Days to Retain Completed PO" setting in PO Options.

What software are you using?  Sage 100 doesn't have a table called PO_InvoiceHistoryHeader.

This is Sage 100 Premium and I wrote the wrong table name. I meant to write PO_PurchaseOrderHistoryHeader. The days to retain is set to 365 days. 

Then the PO tab should show open PO's, and PO's completed within the past year.

It would seem that the Custom Office "Update Custom Panels to Current Level" is the culprit. I restored the AP\Custom folder, took a look at the AP P/Os tab and the data appeared. I then ran the custom office utility, checked the the tab and the PO's were no longer appearing. I restored the folder again , the POs were visible, so I left well alone.