Solved
[BY DESIGN] Alien and normal sim never have alien babies
Product: The Sims 4 Platform:PC Which language are you playing the game in? English How often does the bug occur? Every time (100%) What is your current game version number? 1.61.15.1020 What...
@comicsforever : This has always been this way, as far as I know. They are considered "alien hybrids" (not a broken kind of hybrid like other occult types who get mixed due to bugs). EDIT: Here's Carl's guide on aliens, but I can't vouch for its accuracy, so take it with a grain of salt: https://www.carls-sims-4-guide.com/lifestates/aliens.php
By the way, since you use mods, MCCC's Occult module allows you to make the game do as you expect with its pregnancy settings.
