Tax Rate area/ Tax Explanation code in Prepayment voucher

Hi,

I am trying to default the tax rate area / tax explanation code setup in the PO's of the P0411/ZJDE0009 version for prepayment voucher when I enter a prepayment voucher which is different from what is setup in the supplier Master.

Scenario -1

After changing the PO value to 1 in tax area as below and keeping the tax rate / Tax area code blank, When I entered the Prepayment Voucher , the system still populates the Tax area  and Tax explanation code from the Supplier Master for both negative and positive pay items. It does not populate blank values in the negative pay item.