Thanks for passing this information on to whoever is working on this now, and I hope you’re enjoying your new project!
This really does seem like a smoking gun to me. It always felt like there was an “extra”/hidden window somewhere that would swallow Alt-Tabs, thus throwing off the expected windows switching/handling.
FWIW, I only had one Hub window on the screen, so the other one was indeed “hidden” somehow (i.e. not visible).
I then had another occurrence of this where a “hidden” Cubase window would show up in the Alt-Tab screen, but that one wasn’t a Hub window, just a small Icon labeled “Cubase something” (I forget the exact Windows title, and it’s hard to take a screen shot when you’re holding down Alt-Tab because the Prt Srcn key doesn’t work in that case 
)
Hopefully this will help your colleages track this down, I bet it’s behind a lot of the windows handling weirdness.