That is correct box 2 cannot be zero as there would be zero interest paid in 2021 on a loan that had zero principal balance in 2021. Since the date in box 3 is 12/29/2020, the Box 2 amount should be the mortgage principal amount on 1 Jan 2021.
With such a small difference between the loan date and 1 Jan, the amount in box 2 should be the amount of the new loan . You can look on your closing documents or contact the mortgage holder to get the amount.
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