Payable Invoice Discount calculation showing wrong for before validation and after validation.

Summary:

There is a payment term with installment of 1st line for 80% and 2nd line for 20%. And there is a discount of 3% for the 2nd line. And Manage Invoice Option has defined Discount for All invoice lines & Exclude tax from calculation.
Now when I am creating Standard Invoice, with invoice amount of 1150 for the line amount of 1000 and tax rate with 15%, it is showing correct Discountable Amount of 1000 before validation. but after validation it showing of discount amount 800, why?

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