It might be something else. I tried setting to true and have 100% chance of spawning, but still could have them spawn.
I was on a biome (the pumpkin-like biome from Oh The Biomes You'll Go) in a Creative World where the chance of a megaboss spawning was 100% and the possible spawns was 50% Pidgeot and 50% Ampharos. They never managed to spawn, even with 100% chance and aggression set to true (so I don't know if that was the case or not, or even if the fact that it was on an already existing world has anything to do with it)
I already told this to the devs on the Discord, but wanted to comment here as well to have this information in the bug tracker itself too.