Interestingly, my 64F8500 developed EXACTLY this problem on March 3, when it was less than a month old. I didn't notice this thread, because the last update before today was February 15, plus it took me a few days to realize that it went away with the Black Optimizer set to Off or Bright Room.
The very strange thing about this is that the first post in this thread was last August 13.
The service tech told me that he had never seen or heard of such a problem before. He spent a lot of time talking to Samsung, and he told me that Samsung had never seen or heard of this problem before, and had no idea what causes it, or how to fix it. Samsung had him take pictures with his cell phone and send them to Samsung, so they could see it for themselves. He said they were completely baffled.
Swapping in a new main board did not fix it.
After multiple service visits over a period of nearly a month, Samsung finally threw in the towel and offered me a refund or an exchange.
My new 64F85400 is being delivered on Monday.
Counting myself, there's four people just in this thread who have seen this problem. I wonder how common it actually is? If this has been getting reported since at least August, it seems like the right hand at Samsung does not know what the left hand is doing.
When the person on the phone at Best Buy was arranging the delivery, out of the blue she said "Those are really great televisions, when they work".