>>18485595That's just interframe compression going too long without any I-frames (the complete "real frames") and letting the P-frames (which only store the difference between it and the last I-frame) accumulate artifacts on the moving pixels.
This kind of issue used to be common back in the early days of digital video because storage space and Internet bandwidth/speeds were very limited and they had to crank the compression all the way up to make the files small enough to be manageable, at the cost of artifacts like that.
Nowadays it's very rare because we have cheap giant drives and infinitely better Internet connections.