What's your game's population data?

@Daravi The occult population only includes living sims. This is pure speculation, but my guess is that they originally spawned in your game as adults, and they're now dead of old age. In the first save I shared, my gen 2 went to Sixam a few times, met some aliens, and had some pure alien babies. Her alien kids never reproduced, so now there's no more aliens/hybrids in my sim count. The game theoretically only spawns sims if no existing sims can fill a role, so unless you go to alien bar night or do scientist/alien things, the game won't spawn any more. There had originally been mermaids and spellcasters in my save, but age/culling got to them and they're no more.

It's also possible that there's some MCCC settings under either occult or population that are preventing aliens from spawning. I have definitely mucked with those settings before and had results I wasn't expecting.