Are semi-megabeasts included in the megabeast check? It doesn't make intuitive sense, but could account for the discrepancies you've described.
Good question! I did some more testing and have found three possible outcomes (with default threshold of 80%):
- There are plenty of megabeasts, some die, some survive, and the check does not occur during the test time. The population of megabeasts never drops below 20% of their starting amount in any of the tests.
- There are three or fewer powers (i.e. megabeasts + titans) in the world. Even if not a single creature died yet, the megabeast stoppage occurs.
- There are plenty of titans but only one or two megabeast in the world. In two cases, as soon as the megabeast die, the megabeast stoppage occurs, even if semimegabeasts are still alive. This suggests that the titans are ignored. However, in a third case, the megabeast and three out of five titans died, yet the world kept going. (At that point, I remembered about a raw mod that has a night troll producing semimegabeast spouses. Said trolls didn't produce any semimegabeasts in the stoppage cases, but produced plenty in the last case.)
In conclusion, I am not sure if semimegabeasts matter. Perhaps they only count as a fraction of a megabeast? Regardless, I believe the "opposite problem" I had earlier was caused by having to few megabeasts/titans in the world to begin with, which seems to trigger stoppage anyways.