Please, do not be angry if this is an old issue, I saw it today for the first time.

Pressing volume up/down shows a popup, and closes it with a slow fade out animation. And that animation gets flickery if it is only half over a youtube video.
I couldn't record it properly via some software, so I used my phone, but even there, you can only really see it at the very end.

https://youtu.be/D9a4gF04hIU

Have fun!