I'm a fellow user, not a tax expert, but I wonder if it's this. Do you have other capital gains and qualified dividends that invoked the use of the QDCGT Worksheet for your tax calculation?
If so, the Qualified Dividends and Capital Gains Tax worksheet uses the IRS Tax Tables (if income less than $100,000) for the final calculation to help it to determine which is the lower rate. At your income level the IRS Tax Tables are in $50 increments. Think of them as $50 brackets. If you are near the top of one $50 bracket and add a few dollars of income, it can push you into the next $50 bracket of the IRS Tax Table.
Look at the QDCGT Worksheet in Forms Mode and the calculations on the last 4-5 lines. Specifically, look at Lines 22 and 24 of that worksheet. Then look up the tax figure shown there in the IRS Tax Tables, which you can find in this IRS Publication, and you'll see how the $50 "brackets" would work with your income figures.
https://www.irs.gov/pub/irs-pdf/p1040.pdf
[EDIT NOTE: 2/13/2026 at 6:10 PM Pacific Link revised from the IRS HTML web version to an IRS PDF version, which is much easier to read.]
Try looking at the QDCGT Worksheet and the IRS Tax Table amounts both before you add the additional interest, and look at the QDCGT Worksheet and IRS Tax Table after you enter the extra interest to see how it changed in regard to the IRS Tax Table $50 "brackets." You may have simply been about $3.63 below the next $50 bracket, and if so, then the extra $3.69 sent you over the cliff into the next $50 bracket in the IRS Tax Table.
