patrickbennet, your method is likely flawed. You must test with a tax return that has nothing but ordinary income (no capital gains or dividends, no Social Security income, no tax credits, no itemized deductions that might be subject to phase-out). If you test with a tax return that has *only* 1099-INT income and test with increments of a multiple of $100, you'll see that the calculated marginal tax rate matches that of the 2018 tax bracket into which the marginal income falls. This is what I see when I test using the appropriate method.
Others saying that they are seeing the wrong tax rate are almost certainly using similarly flawed methods. These questions are asked every year. It's just that this year there has been a change in the tax brackets, so it's easy to assume that one is seeing evidence of a failure to change to the new tax rates when that's really not the case.
Examine your tax return to see if the additional $100 of 1099-INT income is resulting in something other than a $100 increment in taxable income. Examine your tax return to see if your tax is being calculated using a method other than tax tables, such a the Schedule D Tax Worksheet or the Qualified Dividends and Capital Gain Tax Worksheet. Examine your tax return to see if tax credits are being affected by the additional income.
