The brightness problem

Hello, recently I started having a issue. My iPhone automatically dims or brighten the screen automatically. That’s starting make my a little bit annoyed, because I can’t increase or reduce brightness. I tried to found a issue in settings and turned off the auto brightness, but it doesn’t work. Is there any thing to turn off that feature, or is this a some sort a bug? Btw I am using iPhone 7 (iOS 14.4)