Unfortunately, the Premium Tax Credit is calculated on an annual basis, not a monthly basis. So even though you changed your income amount half way through the year, you had received Advanced Premium Tax Credit based on that original number for the first half of the year.
From what you have indicated, you received the Advanced Premium Tax Credit for each month of 2019 and had Medicare for half of the year. You would not have qualified for a Premium Tax Credit if you were on Medicare. When you file your return the Premium Tax Credit is reconciled against your actual income and situation. If you were overpaid Advanced Premiums, you do have to pay this back.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.