Unauthorized Debit order date

For the first time in my life I have had to be disappointed by FNB. I have a personal loan account which I agreed to pay over 6 month on a specific date (last day 31st of the month) When I got up I saw I got an sms form FNB that an amount was debited for my personal loan today 26 AUG 2021. I have been trying to contact their Perosnal loan customer care number *** however I have been on hold for over an hour now. My account is now on a Minus balance because of this debit order, I would like to know why I was debited on the 26th instead of the authorized date 31st??? Will the bank be liable for the amount that is now sitting on a minus?