I have somewhat opposite problem, but since it's no doubt to do with the same thing, aka the chance percentage of the occult genetics passing on, adding it here.
I recently (January 2021) started a fresh save for a 100 baby challenge. I have all the packs. I use two mods: SimsInfo and the one that removes the mosaic blur, neither of which modifies or controls sim behavior.
Since it's the 100 baby challenge, the fathers are never a part of the household, so it's just the mother and the children.
So far the human matriarch has given birth to 20 babies.
The mermaid father did produce a mermaid child.
The vampire father (Caleb Vatore) produced a human child.
The spellcaster fathers (four) all produced human children (with Weak Bloodline trait).
The two alien fathers produced hybrid/human children, aka human birth certificates, no alien form, no powers, but blue skin on one of the children.
Maybe it's just unlucky rolls of dice for them, since that's not a big amount of proof yet, but since I'm quite used to what was described before, aka occults always making occult babies, I've been shocked at this matriarch's stubborn human genes.
Maybe it means that the issue that the original post was about has been fixed now? ...and possibly the chance slider moved a bit too much toward human, this time.
