The Meaning of Life, the Universe, and Everything.
i just noticed in the unconformed the withers will attack a player even if they're invisible, while i haven't tried this myself i have noticed other mobs do attack. several time (just on one map) i've had creepers explode while i was invisible...had to fix the same section of stairs multiple times. i've had zombies chase me from time to time (but i wasn't paying that close attention to them), but the one other one i noticed the most was skeletons. i thought it was odd the first time i got shot at by them while invisible, still think it's odd, but mostly ignore it (not that they can hurt me any ways).
dunno about creative, but this all happend in survival (with host privileges on), ranging from easy to hard. it's also happend on several friends maps that i've been on.
When my family and I play splitscreen multiplayer, the mobs will randomly disappear and reappear as players respawn or go through a portal. This is all mobs both friendly and hostle. New visible mobs will spawn while some mobs are still invisible.
There's nothing quite like trying to fight an invisible skeleton. Arrows seem to come out of nowhere.