Sounds like a bench test. How big are the active objects? How many animations per active? I know you are a veteran of MMF use, but MMF2 is not in the habit of code breaking at random.
I think that how many objects per frame you set it to, MMF2 reserves space in memory. I agree that it is not a good idea to just to go off into the sunset and set the frame limit to 20,000 actives.
How big is the backdrop(s)?
How about uploading an .mfa so we can see this random bust of code that MMF2 is doing.