That is LED panel failure I'm sorry to say. Happened to me years ago on the one and only Samsung TV I purchased. With panel failure it'll cost more to replace the panel than it would to buy a new TV. Needless to say I've never purchased another Samsung TV since or anything Samsung for that matter, until I recently bought a new phone by Samsung. More fool me most likely 🤦...Google Samsung Panel Failure it's extremely common, Samsung are well aware of it but do nothing because 90%+ of the time when it happens the TV is conveniently out of warranty. Funny how it always happens once the warranty has expired, don't you think 🤔