When the new 'Tax calculation' functionality is enabled and sales tax code setup of calculation method field is set to interval, the native logic for tier tax calculation should be followed. Currently, the tax calculation always uses the rate that matches the invoice amount regardless of the interval setup on field calculation method of the sales tax code and does not follow the logic described on this link under 'Example: Interval method of calculation': About the Whole amount and Interval options for sales tax codes | Microsoft Learn