Before you accept the seasonal explanation, measure the warm side surface with a probe or an infrared gun. Not a stick-on dial, they are decorative.
They need surface heat to digest, and a mat that was holding 31C in a warm flat in August can be sitting at 25C in a cold one in October with nothing having failed and no setting having changed. Appetite follows the warm spot down. Mine brumated two winters running and it turned out to be the room both times.