Hi- I have had this exact same issue with my 2019 CHR petrol 2WD. I have had my engine cleaned a total of 5 times in a period of twelve months - basically every 5000kms. There is undoubtedly an issue with the engine design. My car started having the problem at around 63,000km. Toyota have advised now that the car needs a full engine clean as part of his maintenance on an annual basis - pretty sure if buyers were told this they would have asked why and not bought the vehicle.