Hello, what you are covering is the proximity sensor, not the light one. The proximity sensor turns off the screen when covered, as the system think it has been inserted in a pocket or inside a closed area, so you dont need the standby screen. Thats the normal behaviour and it's made like that to preserve the battery. Also unlocking by double tapping the screen doesn't work while this sensor is covered.

But you may have a faulty light sensor and the fixed brightness could be a simptom of that. Try reflashing the device and if that doesn't work then you may have to replace the module.

Good luck! Anyone more experienced than me can help you better i'm sure.