Hugh,
According to the CFPB at this link, interest is charged like a credit card, based on last month’s balance. The lesson here is to read your loan documents. It doesn’t matter what you are told, it only matters what is in the documents that you sign.
“Each month, interest and mortgage insurance charges are calculated based on the current loan balance. These charges are added to your loan balance. The amount you pay in interest and mortgage insurance compounds the same way a balance on a credit card does. The loan balance used to calculate interest and mortgage insurance charges each month includes prior months’ interest and mortgage insurance charges. As your loan balance grows, the amount of the interest and mortgage insurance charged that month also grows.”– J.M.