There is a not-obvious relationship between the SEP-IRA deduction (line 15 of Schedule 1 (1040)), the self-employed health insurance deduction (line 16 on the same form), and any premium tax credit. Sometimes, increasing one appears to increase your tax for seemingly unknown reasons.
Look at your 1040 and Schedule 1, 2, and 3 to see what changes when you add the SEP-IRA contribution. I bet something else is changing. Let us know what.
If you are using the Online product, you can see your 1040 and these three schedules at any time by clicking on Tax Tools on the left, then Tools underneath that, then View Tax Summary in the center, then Preview my 1040 back on the left.
If you are using the CD/download software, you can see all your forms at any time by clicking on Forms on the upper right.
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