That prepaid interest should have lowered your starting balance for second lender. Check first statement and see starting balance and look at first lender if you have any access to them for closing balance.
First lender worksheet should show interest you paid and 0 principal. The end balance should be less the interest paid. Second lender origination amount would be first lenders end balance and the second lender would have end balance based on your last statement after final years payment. Sounds to me like you need a corrected 1098.
Look at info for your current lender and see total principal paid, interest paid, and the year end balance..use those numbers to work backwards to get your starting origination balance for second lender.
