OK, supporting documentation I’ve found so far:
Per Broan “You should never exhaust air into spaces within walls, ceilings, attics, crawl spaces or garages. The humidity and grease collect in the space.”
FAQ
Still a little thin though, as most microwave hoods can be configured to exhaust back into the house (kitchen).
Nagging at the back of my mind is the suspicion that this must violate some general HVAC prohibition regarding exchange of conditioned air between the habitable space and the garage...