FNB Personal Loan

On the 15th of July I paid up my personal loan with FNB as per the balance showing on my FNB online app , to my surprise on the 26th of July FNB debit my account with the interest amount , I queried this with my personal banker and she told me that I need to pay this interest as per the FNB Personal loan department , I have been banking with FNB for a good 14 years and I have settled my loans before using the balance showing on the FNB Online app , why is it a problem now ??? FNB is robbing people but this is the last time because my husband and I will be definitely closing our accounts .