VW SA Maintenance plan ****

My 2021 Tiguan started giving me problems at about 50 000 KM’s, with the EPC & engine light coming on when accelerating. Took the car to a dealership, they said they sorted it out, but the problem kept reoccurring. On the last visit to the dealership, the service advisor told me that VW declined to to change the spark plugs ( car with a maintenance plan ) and their reason was that my car had dirty fuel( after all South African fuel quality is not the best. My query was, how is SA’s fuel quality my fault and also what is the purpose of a maintenance plan(which I paid for) , if it’s not going to help me keep the car in working condition? They kept giving vague responses , but clearly refusing to charge my maintenance plan for the change of the spark plugs. Worst service ever, pity I loved the car.