Strange display issue on FaceBook and YouTube

Hi all,
New user here. Since a couple of weeks I see a strange intermittent feature when scrolling through Facebook and YouTube. Sometimes a couple of lines near the bottom are displayed really strangely, see attached YT screenshot.

After a few seconds, and/or when scrolling further up, they become normal again. I can't reproduce it at will, it just randomly happens every now and then, always near the middle of the bottom of the screen. No idea what I could have done to cause it. It's not much of a problem, just mildly annoying. Be interested if anyone else has seen this and knows more. I'm on Windows 10 Pro Version 21H2 (OS Build 19044.2251)
Any ideas welcome, thanks !
Chris