MTN *******

I canceled 3 contracts in Feb 2023 and wasn't billed anything extra, was billed for the remaining contracts I had, in June I canceled another contract and yesterday MTN bills me for it, I called them and they are telling me cancelation takes 30 days, I get that part,but the contract ended, why am I being charged the full DO and why wasn't I billed like this in Feb when I canceled my other contracts? MTN is a disgrace honestly!