I've applied a glass screen protector a few days ago and found that the sensor on this phone is so sensitive that it picking up the reflection from the screen protector, because the sensor is right under the display so it's not possible to have a cutout for it.
You can test a proximity sensor on this phone with *#77692# code, if it stuck in green and vibrates after you press "Proximity On" it means it's thought that the sensor is being obstructed, this will be very annoying when you making a call and screen doesn't turn on automatically after you pull it away from your ears.
What I've done is to reflash stock rom with Odin, checked the "Nand erase all" (This will erase all the /data partition, basically factory reset the phone), when it comes up it has saved the new Proximity sensor baseline reading and everything function correctly again.
You can test a proximity sensor on this phone with *#77692# code, if it stuck in green and vibrates after you press "Proximity On" it means it's thought that the sensor is being obstructed, this will be very annoying when you making a call and screen doesn't turn on automatically after you pull it away from your ears.
What I've done is to reflash stock rom with Odin, checked the "Nand erase all" (This will erase all the /data partition, basically factory reset the phone), when it comes up it has saved the new Proximity sensor baseline reading and everything function correctly again.
