Adaptive brightness function is failed/delayed.

1. First of all, check Settings-Display-Adaptive brightness switch. If it's off, please switch it on. 

2. Please try to use the device in a different light condition or environment.

3. Open [Settings] - [Display] to check whether Reading mode or Night mode is enabled.

4. If the issue still persist try to clear cache, back up all the data and flash to the latest OS version.

5. If actions above are not helpful, please contact OnePlus after-sale support.