Bad customer service from FNB call center.

I am very disappointed with FNB and the customer service is disgraceful. I had taken a personal loan with them and every month the payable amount is calculated correctly, but last month was the final payment for the loan and the amount calculated was less, I never think anything wrong as it was the final loan amount and the amount calculated is by the bank. The payment came out on the 30th of April 2025 and it was not updated on my banking app that the payment was taken, I called the call center and spoke to various departments and they told me to wait 3 working days for the payment amount to update and if not updated to call back after 3 working days. I had called them after 3 working days that is the 3rd of May 2025 because of the holiday in between and spoke to the collection department and they advised me that the system calculated incorrectly and should pay the balance. I asked them as they have 1 month after the last payment to calculate the next payment and nobody saw that there was an issue with the payments been incorrect. The advisor just told them only once the payment comes out and when the customer calls in then they only find out that the payment was incorrect and they just advise the customer and there is nothing they can do, no apology, no compesation just that you have to pay the amount if you want further assistance. I asked to speak to a manager and they advised no management is in on a Saturday. I wanted to take another loan and can not now because of the system providing the incorrect payment amounts and the customer has to suffer. I am really disappointed and unhappy and something needs to be done about this from the bank.