It sounds like the LCD/OLED is aging. Let's look at when the V20 came out.
https://www.gsmarena.com/lg_v20-8238.php
About 2 years old so that's about how long smart phones seem to last til issues.
Again, because this fades in an hour or so it sounds just like "old screen" issues.
I have an LG V20 and I like to take long videos with the phone, but what I noticed after I am done recording and use it for everyday use like looking on social media I see a faint overlay of the camera settings. It looks like it burned into the screen but after an hour or so camera settings are no longer there. So my question is can I turn off the screen while recording or will turning the screen brightness lowered all the way help?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic