I have seen multiple people face this issue, this is not something you could fix, its a screen problem, I had it once, a lot of people had it, I can tell you now, if you want to get it fixed you have to get the screen replaced, if you are covered under warranty then you should be fine to get it fixed.
I can tell you why it happened and how you could prevent it from getting worse, or if you fixed it not to let it happen again.
This issue is caused due to one of the following options or both together
2) Applying pressure on the phone (being in your pocket while you sit etc...)
I can advise you not to use the phone while charging as it might cause overheating which could lead to this issue you are facing
most likely its the second option with most phones, as people do not pay attention when they apply pressure on their phone, that being said, sitting while your phone in your pocket could be applying a little pressure on the screen (bending it) and with time, this issue could appear and will get worse if you are not causious, I always take my phone out of my pocket and place it on a flat serfice freely, I always make sure it doesnt over heat, if it does I immediately stop using it and try to let it cool down.
By doing so, its been more than year and I had no problem with the screen...
these little things that people dont pay attention to (like myself) are the things that could lead to shorter lifespan of healthy hardware.