One Tax Code maintained as default tax code in SRM

One Tax Code maintained as default tax code in SRM, but at the time of creation Purchase Order, a different tax code is chosen in PO, but after approval of such PO, it is automatically converted into default tax code in such PO.

Tax Code      Rate                    No Tax          Default

A0                VAT 0%                    Y                    Y

4A                VAT 14.5%              -                      -

Before approval of PO  Tax Code "4A" but after approval tax code converted into "A0".

How can I overcome from this problem, please suggest.