Purchase Order [ast_po] and Line Items [ast_po_item] tables used to be a part of Contract Management plugin [com.snc.contract_management] but they are now deprecated. The upgrade might not have remove these tables if they were customized or other reasons that could be specific to instance.
If these tables are critical and if these tables can not be deleted and you would like to update/migrate to the latest version . Procurement plugin has 2 new tables proc_po and proc_po_item which have a very similar schema as ast_po and ast_po_item tables. After activating the plugin data & customizations can be migrated to proc_po and proc_po_item tables.
Note: All changes should be tested thoroughly in a sub-prod before making changes in prod.