Every time an Occult and Human have A Baby

I tend to always pair mine with a spellcaster because I love the option of a normal born sim born with a bloodline trait it kind of gives it a hybrid feel without breaking the game. Though in your case OP I think it is mostly due to the genetics or the previous generations. If one of the occult sims is with a normal sim then yes it would be a 50/50 chance even if the parent was inpregnanted before the other was turned either by kelp fruit, vampire bite or through a spell but after that there would be a 50/50 chance then the offspring would be occult. However what I have noticed though is that the spellcaster is a little more dominate than the others due to the bloodline being passed down. It would be a total 1/50 chance that the offspring would become normal.
Though with MCC I think you maybe having more of a population issue so you could just go into it's settings and tweak it so you could have a normal sim at some point or you could just use the occult command and human-ize the sim and then go from there.