MTN SA ******** expired contracts

My contract was eligible for cancelation on 27 of April 2021 and I paid the amount owing on the 26 of April 2021 then called their retentions department on the 28 of April 2021 to cancel the contract, I received an sms with a refence number stating that my case was closed. On the 14 of May 2021 I received an invoice and statement stating that I owe MTN SA for data recharge on the 12 of May 2021 by MTN SA yet I had already canceled the contract. On the 14 May 2021 I called their retention department and they told me my contract wasn't canceled and only today did I receive an sms stating my contract is due to for cancelation. I would like to ask MTN SA to explain why am I being billed for a canceled contract because the fault was on their side and not mine.