It is likely you have a tax credit that is sensitive to decreases in income, such as an earned income credit for instance. As your income decreases, the credit decreases more that the decrease in your tax so your tax due goes up.
You will need to look at your Form 1040 and schedules 1 to 3 before and after you enter your retirement plan contribution to see what is changing.
To view your form 1040 and schedule 1 to 3:
Choose Tax Tools from your left menu bar in TurboTax Online while working in your program
Choose Tools
Choose View Tax Summary
See the Preview my 1040 option in the left menu bar and click on it
Choose the Back option in the left menu bar when you are done
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