Inaccurate wording there that can be mis-interpreted. The question is, "is the outstanding balance on the loan as shown in box 2 of the 1098, more than $750,000?"
Yes, and I was actually just reading that mortgages and refinances taken out before December 16, 2017 are grandfathered in at the $1 million debt limit. So, that would not be a reason for the decrease.
I wonder how much of a decrease is involved here. The amount of mortgage interest paid each year will go down and should go down with each passing year, as the outstanding principle loan balance goes down. They're asking why their mortgage interest when down, so I'm not certain if they're talking about the mortgage interest actually paid, or something else.