What is causing the a phone's LCD to develop these ugly bright spots?

As an avid 2nd hand phone buyer this is something I have come across this issue many times. A phone with an LCD that for some reason has one of these distracting bright dots or brighter areas in the display. I also have noticed that on certain models and manufacturer this problem is especially common. On Sony Xperia phones from back in the day when they still used LCD 6 / 8 phones I own have this issue to some extend. Also on it seems to be very common on the LG G5 and the Huawei Mate 20 and P20 for some reason. But that being said, iPhones and HTC phones also seem affected but much less often.

The question that I have is - what is causing this so I can avoid it in the future. I personally never have managed to do this kind of damage to a phone that I own, I have only seen it on phones I bought used. I also don't think its pressure. Because pressuremarks look definitely different on an LCD (by discoloring parts of the display, not making it brighter in a certain spot)

Also if there is a fix for it that doesn't require changing the display it would be amazing.