factory reset didn't make a difference. but what i did realise is that using the stock case and using it naked will make the proximity sensor work properly. I've got 2 cases and they are hard plastic type, both causes this issue to happen, oh well.. have to live with it or i change to those thinner TPU cases or maybe change to the XUNDD one since u're using that and it's fine..
doesnt make sense to me since ours is budget phone but I think our proximity sensor is the ultrasonic type.. else how on earth when i press the back of the phone, the display will turn off.