<<Apparently, it has something to do with the premium tax credit.>>
I don't know if this is your problem, but if you received advance credit payments toward the Premium Tax Credit, and the IRS subsequently determined that you were not eligible due to the household income on your tax return, they would require you to repay those payments.
https://www.irs.gov/affordable-care-act/individuals-and-families/eligibility-for-the-premium-tax-cre...
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
