Please check the current status of the PO through Inquire -> View Action History
If you 'd like PO to be automatically approved after modification.
Please refer to How Reapproval Rules Can Cause Purchasing Documents To Require Reapproval? (Doc ID 1114515.1)
The Rapproval Rule can make the PO approved automatically without submit to PO Workflow Approval.
Pls run the Concurrent "Workflow Background Process" select the item type as "PO Approval" also enable the process deferred & Process timeout as "yes', once complete then check the PO Approval status.