strange partial dim display issue on the iPhone6S+

[zoran]

On the right hand side of my iPhone's 6S+ display, i noticed a partial dim.
Im attaching a photo so you can have a look how it looks like. Im close to having a heart attack! Is it a known issue? Ive searched but cannot find anything similar!
Please help guys!

 

[zoran]

Suppose i do a hard reset, is there something i need to backup?

 

[zoran]

One day i noticed that this partial dim did not occur after a restart i made. This happened only once but i guess its worth mentioning. After a few seconds of use though it was back again.
I dont know if its relevant but i can no longer hear the click/tap sound when i enter the phones lock code (im not using touch ID), or might this sound absence be a setting?