Turn off Allow Tax Applicability but now no Tax is calculated

Hi

We have the following issue:

We use Procure to Pay, with Oracle iProcurement and Oracle Purchasing.

We create a Requisition and enter the Tax Calssification Code on the Req.

The Req is submitted and Approved.

When we auto-create the Req, the Tax Code is still correct.

As soon as we make any change to the PO header, lines, shipment and distributions the Tax engine kicks off again and replaces the Tax Code with the Tax default.

As a workaround, I have turned off "Allow Tax Applicability" for the event type "Purchase Order and Agreement ".

If I now create a new req, and then autocreate the req into a PO and then make changes to the PO the Tac Code Remians the same but it doesn't recongnise the Tax Code as does no calculation at all?