It’s an unfortunate effect of H.264 video compression (or any similar method). It works by only sending an initial full image frame, and then only updates to that frame. e.g. Set all pixels to this image, Move this pixel 10 to the left, this one 6 to the right. Rotate this set, change these pixels white, etc, Repeat.
If that initial frame or an update frame is dropped, you get to see how the image compression actually moves the pixels around.
There’s ways to avoid glitches like this, such as sending initial frames more often, but obviously none of that happened here.