Then go to Options (hamburger icon at far upper right side or ALT key-> Tools menu -> Options). Scroll down in General (which should be the default view) to Performance and uncheck Use Recommended Performance Settings. That will reveal Use Hardware Acceleration When Available, which will probably be checked. Uncheck it.
I now have normal YouTube playback again. Thanks to philipp_sumo on Reddit who gave me a trail to follow for this fix.
This bug seems prevalent in AMD graphics. Updating the driver didn’t help me. The above did.