Give me my refund as promised in writing!!!!

Absolutely shocking service from KFC. Firstly after numerous occasions of not receiving my full order from my local KFC when ordering on THEIR app, this happens again on July 11 2025. I call in to customer support and they apologise and offer me a full refund after canceling my order. The following day, 12 July 2025, I received an email CONFIRMING MY ORDER CANCELLATION AND REFUND for the FULL AMOUNT OF MY ORDER, they have only refunded me 10% eventually after almost a month. Then, when asking why the full refund was not issued, they have no explanation other than not knowing why I received WRITTEN CONFIRMATION of this order cancellation and refund. Apparently it is some "system error". Unacceptable as this is binding as written confirmation of the refund and now they say they can't issue the refund?!!