Incorrect Billing

I upgraded last year from My MTN 10GB to My MTN 20GB at R499 a month. I still phoned back end of August because I wanted to know if iI cant go on to the R299 special

All off a sudden when I logged into the My MTN app on (08/06/2020) and saw my balance is R1997.44. Your customer care said that all off a sudden my discount for the contract is not applicable anymore, which I believe is untrue. It's now the end of June and this incorrect debit order went of my account and still no help was given to me