I have just returned mine today. The same experience with the Apple Store and the premium reseller from my country. They said that they did not receive any reports about this issue until now. During the weekend I took photos with the phone to see if the problem persists and I have compared it with my 3 year old phone a Huawei P20Pro and in every occasion, except the portrait mode, the P20 Pro produced better looking photos, better white balance, better contrast, while the 13PM had mushy colors and sort of a grain overlayed. I also checked the GSMarena review especially the Photo compare tool where you can check to make an opinion for yourself. I am really sorry because otherwise the phone is awesome, especially the latest iOS. Good luck with your devices and I hope this is going to be sorted out.