iPhone trade in program

I signed up for Apple’s trade in program in 2020. I kept my iPhone 11 for over a year (almost 2) and had made the 12 monthly payments required to trade it in. I just traded my iPhone 11 in and got a new iPhone 13 a month ago. But I am still being billed for the iPhone 11 I traded in and now the iPhone 13 as well. I called Comenity bank, and they told me I still had a balance on the iPhone 11, and that once it was paid off I wouldn’t have to make the payments anymore… But how can there still be a balance on a phone I no longer own and traded in?