Ok I actually have an update on this thread.
I know some people found issues with their AJA I/O cards and such, but I don’t have any cards hooked up to my iMac, so i was clueless.
Until today, when I upgraded my 2nd monitor. I used to have a crappy 24″ thing, but now I have a somewhat less crappy 27″ curved monitor (LOVE it so far).
And for some reason, since I hooked it up, this monitoring issue went away. I can now play all apps/browsers simultaneously to playing the Avid.
OMG IT JUST STARTED DOING IT AGAIN!!!!!! No lie as I’m typing this message I looked over and the Monitor button is muted again.

Welp, that was short-lived. lol
Carry on.