XPERIA Z navigation buttons not working

Hello I recently encountered a bug in my XPERIA Z, navigation buttons (back, home, multitask) are not functioning. I noticed that factory resetting phone and setting up phone for the first time the other buttons (Next and Back) are functioning in the same touchscreen area as well some third party apps like dolphin browser navigation bars located at the same spot are also working.

I factory reset the phone, flashed KitKat, flashed Lollipop but also my problem is not fixed.
Any help will be greatly welcomed.

Thank You