VAT amount on Voucher

VAT setup done for PO and AP modules .When I create a new PO, VAT amount calculated and showing on the PO header.

When I copy the voucher using copy PO option, my VAT amount is not getting populated on VAT entered amount. But I click the VAT summary page all calculations are showing. Every time user needs to enter the VAT amount in the Entered amount filed based on calculated amount showing on VAT summary page..

Is it system bug or working as expected ?? Thanks in advance for your help !!