What causes this?

This effect appears randomly to a few of my photos Apple did their tests and found nothing wrong with the phone Just found out that if I go to reddit and select the photo, the photo becomes ok when I go through the reddit photo selection. I needed to screen cap from my iPhone photo and use reddit to select the screenshot in order for me to upload the problematic photo. Apple said they will replace the camera but I am not sure if the camera is the issue? Thanks