It’s definitely not burn-in, it’s likely some kind of defect in the backlighting system. For most LCDs the “backlight” is essentially a big thin white/mirrored panel reflecting or diffusing light in a very carefully consistent way from a very bright light source, typically either a fluorescent tube at the bottom or more commonly nowadays evenly spaced strips of LEDs. Some higher end models use more elaborate designs but they’re the minority. Defects in the backlight panel, the back of the LCD panel, or stuff like dust or even insects getting inside that reflective/diffuser chamber will affect the consistency of the backlight as it both blocks a bit of the light from reaching some places and reflects it to other places it shouldn’t be. That’s what it looks like is happening here. It could be some kind of delamination of some of the surfaces inside the TV, or it could be some puff of dust that somehow got inside, or even something like a spider decided that was a great place for a cobweb. Without opening the panel it’s hard to say what’s going on exactly, it might just need a very delicate cleaning or it might need replacement parts.

If you’re afraid of spiders, I’m sorry, you just have to burn the house down now, it’s the only way to be sure.