The same thing happened to me. I ended up losing the whole dark form of my vampire-mermaid sim trying to fix it... Is there anyway to stop the glitch or are you forced to max the vampire lore skill and make the vampire cure to fix it/eat mermaid kelp twice?
I had a vampire/spellcaster born a week ago and today realized another sim was born around the same time spellcaster/mermaid. You can use MCCC to remove occults and turn them human. There is also a hybrid stabilizer mod that allows you to access them in cas or easily remove any type you don't want. https://modthesims.info/d/647863/all-in-one-occult-hybrid-stabilizer.html
I wish they would fix the Mermaids. They are the only occult trait I can't add onto my sim. I have noticed that only my Alien/Spellcaster/Vampire hybrid sim plays well. FYI, I only use the traits codes cause I only play vanilla.
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https://modthesims.info/d/647863/all-in-one-occult-hybrid-stabilizer.html