Cancelling with MixTelematics is a nightmare!

On 27 March 2025 I notified MixTelematics (the operators of the Matrix tracking system) that I had just sold my car, and I therefore wished to cancel my contract with immediate effect - but they only processed the cancellation on 7 April, and sent me a notification that the cancellation would take effect on 9 May, i.e. 43 days after my notification to them, and more than a month after the processing date. Since my notification I was charged for April and for May, on the pretext that because the cancellation was not not cancelled before the 7th of the month, it could not take effect at the end of that month. While I concede that I should be expected to pay for April (on the basis that a calendar month's notice can reasonably be expected), I cannot understand why I am being billed for May as well. I suspect that this is a case of revenge billing, because I chose not to use Matrix for my new vehicle.