Dimming screen in the sun

Whatever I do, my phone keeps on dimming by itself. I've tried to look it up but I couldn't find my problem anywhere else. All I can find is to turn off auto brightness, zoom and reduce white point. All of these settings are off and my phone keeps on dimming by itself. Worth mentioning is that this only occurs when i'm outside in the sun. So let's say i'm on the beach and I play a game for a little while and I can see perfectly fine. After maybe give or take 10 minutes it's start dimming piece by piece until it's barely possible to see what happens on my screen anymore...

Maybe I didn't look good enough on the forums or I forgot something... But I hope i'll find my answer here. Thanks in advance.