iPhone Screen is too dim in direct sunlight

Not sure how to fix my phone so I can easily see the screen in direct sunlight? I have auto brightness turned off and my brightness all the way up. I restarted my phone and it is nice and bright enough, but within a few seconds it goes dim by itself even though the brightness level is still all the way up. It doesn’t make any sense. Other iPhone users that I’ve been with in the sun have no issues with their phone screens and their phones are bright and still easily readable in the sun. I have to squint and create shade just to be able to see what I’m doing on my phone and it’s frustrating! Please help!