I've tested this under several scenarios and can find no evidence that TurboTax behaves as you describe. How do your AGI and MAGI react when you change your self-employed health insurance deduction to zero?
Is the business under which you've established the health insurance plan a Sole Proprietorship (Schedule C or F), a partnership (Schedule K-1 (Form 1065)) or an S-corp (Schedule K-1 (Form 1120-S)?
Do you have Marketplace insurance?
