Hi Tom-
I spent some time today attempting to reproduce this as well. Much like the other focus issue we recently discussed, I was unsuccessful 
I’ve taken a look at your System Report (thank you) and I don’t see any red-flags.
You mentioned earlier that you often use Rhino on a secondary display. It’s a stretch, but I would be interested to hear if you start to experience these focus issues when you are unplugged and using your MacBook Pro’s built-in display (perhaps a rarity too).