Does anyone know why this is happening?
Looks like a hardware-specific issue, probably related to graphics drivers. I've seen several complaints about this, but nobody has reported a bug yet, and developers won't investigate without a bug report. So I would take this straight to WebKit Bugzilla, following the instructions for reporting web content bugs. You'll probably be asked to provide details of your graphics hardware and drivers, but no need to worry about that until you're asked for it.
Also, if you happen to have gstreamer1.0-vaapi installed, definitely try uninstalling it to see if vaapi is to blame.