FNB loan is a ****

I had a loan of R76000 with FNB. Well, I have been paying the loan without a fail, Last month (August 2024) the loan had a balance of R3191. On the 25 of Sept 2024, I paid it off, immediately after I made the payment, the loan account showed a -R65. I could not understand where does the R65 come from as I have settled the loan, to my surprise today I woke up to -R 381. Why is FNB ****ming me? and this is not the first time, they did the same with the previous loan I had with them, and I cannot be told that it's interest, interest on what when I have settled the loan?