Custom mobs gradually stop spawning (1.18.2)

[sixbullets]

My mobs are all subclasses of PathfinderMob and don't have any spawning restrictions. I added each mob to the respective biomes with weight: 100, minCount: 1, and maxCount: 2. I can send any relevant code if it helps with troubleshooting.

When I first teleport to my custom dimension and run /forge entity list, I get the following:

After I fly ~500 blocks away and run the command again, there are significantly less entities spawning.

If I return to the first location and run the command again, even less entities.

In the vanilla overworld I am seeing anywhere between 200 and 300 entities, so I know there's an issue. But I'm not sure where to start looking. Which files restrict mob spawning?

[sixbullets]

For anyone else encountering this issue: add your mob to the biome's "spawners" list twice: once as a monster and once as a creature.

No idea why that's the fix, but now I'm getting 600 entities to spawn.