MTN incorrect data usage billing

I received my MTN bill where an amount of R565,20 for 'data charges' was included on my bill. I do not normally purchase data as I have a monthly bundle, but during a holiday my data ran out on the 28th of July and I needed to purchase an extra bundle. After several unsuccessful attempts to purchase it via MTN services (I have all the sms' for these attempts) I eventually succeeded in purchasing a data voucher via my FNB banking app for the amount of R499.

I received an sms from MTN saying recharge was successful, yet now it seems that they are charging me for a 'data session' used on the 28th of July 2018? I have checked my bank statements, which show the purchase of the voucher. I have checked my data usage on MTN for the day in question and it shows my data use dropping off around 14:00 as my monthly data bundle was finished and then picking up again after 15:00 when I recharged with the new data bundle. What gives? I do not think that I could have gone into out-of-bundle usage because usually when my data is finished I am unable to access any internet on my phone and I was constantly checking my MTN app to see when my data would run out so that I could purchase a new bundle