I downloaded from the IRS the 2021Instructions for Form 8962. On Page 4 it states the following which may be helpful:
Monthly contribution amount. Your monthly contribution amount is used to calculate your monthly credit amount. It is the amount of your household income you would be responsible for paying as your share of premiums each month if you enrolled in the applicable SLCSP. It is not based on the amount of premiums you paid out of pocket during the year. You will compute your monthly contribution amount in Part I of Form 8962.
