That’s great to know, glad it got you sorted out. When you have IAC buses they can cause some problems with feedback loops (one app gets events and forwards to the bus, another takes the input from the bus and echoes it to the bus, etc), and it’s quite possible that’s what happened here.
I think I now have a fix for this which will be in the next version.
Amazing!
Dr. Walmsley strikes again! 
one observation regarding IAC… on my (W) systems Dorico is set to NOT use any virtual midi cables as Midi input. However, if I start a new project (intended for Midi output only) and remove any VSTs from the rack BEFORE I define my Midi Instruments, Dorico will act up with all sorts of perpetual Midi Loops as soon as playback is started. If I define my Midi Instruments but leave the VST rack untouched, then everything works as expected…
Regarding excessive CPU usage… a case was recently mentioned on the Finale forum and a senior member mentioned that the easy fix was to change the Mac’s color profile… (go figure… you never know… 
)
I think that is less a comment on the interconnectedness of all things and more about the state of Finale’s internals…! (And I’m also somewhat sceptical of that ‘fix’ given the paucity of information.)