How to stop iPhone from automatically dimming the brightness?

- I have unchecked Auto-Brightness under Settings app → General → Accessibility → Display Accommodations. - I have also disabled Night Shift under Settings app → Display & Brightness. - I have Low Power Mode turned off under Settings app → Battery. However, every time I use the iPhone for either more than 5 minutes or use any apps, the brightness automatically dims down. It’s frustrating since I play games and use apps all the time. The brightness keeps fluctuating. As I understand, there are no hardware faults. I believe this is purely a software bug. I would really like some help. I am using an iPhone 7 running iOS 12.3.1.