The Premium Tax Credit ("subsidy") is not based on monthly income, it is based on annual income.

That means that income received in December will affect your total credit that you receive for the year.  If you received too much Advance credit (the amount they paid to reduce your monthly insurance payments), you may need to repay at least part of the Advance credit when you file your tax return.  When you enter your 1095-A into TurboTax, it will calculate if you need to repay anything, or if you will receive additional credit.