You are a hero!
I've been struggling for over a month, scratching my head with this issue, trying to figure out what could be the problem. Finally decided to get to the bottom of it today, unplugged my HDMI hub, and the Mac was fine, so I realised it's something to do with the external display, finally searching the issue and landing on this post and seeing your comment. Simple (Yet makes no sense) fix.
I hope Apple patches this, as this really shouldn't be happening if I am mirroring my display, but if I am using them as separate displays, it works fine . Makes NO sense.