If you are using FlexNet Manager Suite On-Premises then you may be able to find some information about the deleted PO record by querying the the CompilanceHistory view in the compliance database. This view contains one row for each tracked property value change. You may find rows recorded showing a non-NULL OldValue and a NULL NewValue that correspond to the deletion operation.
You will have to dig in to the data and work to identify a query with appropriate filtering to find exactly what you're looking for, but start with a query like the following and work from there based on what it shows:
SELECT TOP 1000 * FROM dbo.ComplianceHistory WHERE PurchaseOrderID IS NOT NULL AND OldValue IS NOT NULL AND NewValue IS NULL ORDER BY ComplianceHistoryDate DESC