Ongoing interest while you are Garnished

I've had a loan with old mutual and I defaulted the account with the balance of R73 341, 52 and was garnished at work that was 2014, and I've been paying since then up until January 2019 that was my last payment , now I was requesting a paid up letter from them they told me I still owe the and I was shocked to hear the news , I asked them I can be that possible because I've been paying them with the hope that they have request all the money I owe them when they garnished me now they tell me its interest , in my understand the interest were included in the amount , she told me they have stopped the interest in 2018 September and I've asked her why they stopped the interest only in 2018 when I'm about to finish the payment they is no response, Hello peter can you please help me