I've seen this a few times, and I think I've only seen it in Beat Saber so it might be a bug in that game. I get a black rectangle right at the front of my field of view, about the full height and 2/3 of the width. It seems to be made of large pixels, around 12x24 pixels in size, with some of them missing, some flickering, and some outside of the rectangle.
It just happened right at the end of a song, and went away a few moments later when the scene switched but then the entire scene went black. I went back to the home and everything there was fine. I tried loading another app and it was ok too. But I tried a few times to get back into Beat Saber, and after the initial logo the scene (presumably the health warning screen) was still black. It's working again after a full reboot.