By "December interest" do you mean the interest that accrued in December and was paid with the mortgage payment on 1/2/2021?

In my experience it's very very rare that the banks get this wrong.

Then the client should contact their mortgage company if they think it's wrong.

Bank of America mails to at least some customers a monthly statement showing interest and principal paid to date, and the amount owed for the next monthly payment.  The statements sent after the December payment warned not to use the annual interest because it would not include anything paid later in the month, but before the end of the year.  In that case, just look for the official document.

Or maybe, the taxpayer thinks a check mailed December 28, but not cashed until January 2, should still count.  

@rbynaker "In my experience it's very very rare that the banks get this wrong."

You might not remember this from a few years ago.

https://procedurallytaxing.com/bank-of-america-faces-lawsuit-over-supposedly-intentional-erroneous-i...