This is a bit old, but for other people who find it now (like myself).
You can get the alternate calculation for the year that you get married, but it will still be based on annual joint income, and not a month by month basis. So even dropping the coverage just prior to marriage will not prevent you from owing (at least some of) the credit back. As another user said, seems to be a weakness in the PTC, not a problem with turbotax.
https://www.verywellhealth.com/how-getting-married-affects-your-premium-tax-credit-5069846
