My iPhone 7 with ios11 randomly reduces brightness to 50%. It has NOTHING to do with auto brightness. I know it's at 50% because when this happens I attempt to adjust the slider and there is no change in brightness from 50% to 100%. I've tried toggling auto brightness to see if it will reset, I have tried restarting my phone, neither works. This happens very often while in very bright outdoor sun-lit areas and suddenly I can't see anything on the screen.
To make sure, that this is not software related and your light sensor is working correctly, set it back to factory settings, without using any backup data. Then test your phone.
If the sensor works again as expected, use the latest backup to restore your data and settings again.
An encrypted backup made in iTunes before you start, will contain all your passwords for accounts and Wi-Fi.
Restore your iPhone, iPad or iPod touch to factory settings
Any case around the phone, that might cover the sensor on top of the screen?
Any parts, that have been replaced recently, like display or battery?
If setting it back to factory settings did not bring this function back, then the phone has to be diagnosed by Apple or an Apple Authorized Service Provider next to your place.
This article has more info about that:
Didn’t work. But it’s not the light sensor. I have auto brightness off. It will randomly go to 50 then back to 100 after like 5 min
Brightness won't go above 50%
