When tax invoice journal is posted, GST calculation was incorrect

Case 1 -

When a tax invoice journal is entered under Accounts Payable -> Invoices -> Tax Invoice Journal, which posts to a ledger account with a project account as the offset (GST group is selected as GST, and Item GST groups is also selected as GST), GST calculation is incorrect.

The total invoice amount is adding up the item/service amount and the GST amount. Instead, the total invoice amount should be the difference of item/service amount and the GST amount.

Case 2 -

When a tax invoice journal is entered under Accounts Payable -> Invoices -> Tax Invoice Journal, which posts to a ledger account with a vendor account as the offset (GST group is selected as GST, and Item GST groups is also selected as GST), GST calculation is correct.

The total invoice amount is the difference of item/service amount and the GST amount (Case 2).