I have had 3 different iPhones in the last 5 years. 8plus, 12, now 13. (Last two I have bought refurbished as I don’t want or need the latest model). Every one has randomly turned the brightness up and down by itself. I have checked the settings and I don’t have it switched to Auto brightness. Even at its max brightness, I struggle to see it when outside at times. Is it a heat issue or a design fault? Drives me nuts!