AP: Imported Invoice Line with Zero Tax Classification creates default tax line in Invoice workbench

I have imported one invoice with an Item line and a tax line.

The import process is successful and Item line and Tax line has been generated along with an additional tax line .

On clicking on the calculate tax button , the default tax vat16 is getting calculated again as the default tax classification code is assigned to item line by default from the supplier site.

CALC_TAX_DURING_IMPORT_FLAG  and ADD_TAX_TO_INV_AMT_FLAG are set to NULL

Tax Manager -> Defaults and Control -> Application Tax Options ->

1) Supplier Site

2) Supplier

How to avoid further tax calculation once the invoice is imported