Hello
Here are a few suggestions that might help:
Check Power Mode: Open Settings > Power > Power mode. If your device is set to Best power efficiency, it may reduce performance to save the battery and screen brightness will also be reduced. Set it to Balanced or Best performance.
Disable Adaptive Brightness: Open your active power plan, you’ll see Change plan settings option. Click Change plan settings next to your current power plan to open Edit Plan Settings window. Click Change advanced power settings option. This will open Power Options dialog. Expand Display and then expand Enable adaptive brightness.
Try Disconnecting and Reconnecting Power Supply: Open any content that is bright, like a blank page. Disconnect the power supply. After a few seconds reconnect power supply.