I was curious about this, too, since I plan to have mixed human and occult families. I don't mind if the children are fully one occult or the other, as long as they're not a glitchy hybrid.
I was googling the hybrid glitch issues with mermaid hybrids found this thread. It said supposedly the mermaid hybrid issue (with maybe the spellcaster?) were patched-fixed to be either one or the other...? (No hybrids.)
This thread from 2020:
https://answers.ea.com/t5/Bug-Reports/FIXED-IL-ROM-GTW-Hybirds-Patch-1-58-63/m-p/8438782/thread-id/110315
"Mermaids and other Occults should no longer be able to make hybrid occults. How were they making them? Oh boy, I'm not sure if it's appropriate for me to give you the mermaid-birds and spellcaster-bees talk."
"Fixed in Patch 1.69.57/ 1.35:
If you're still experiencing this issue, please create a new thread.
Be aware that already existing hybrids will NOT be fixed.
You should however not getting new ones."
I hope they fixed it because I want to have my occult couples to make babies who can be one or the other. (I don't care if they're not hybrid, as long as they don't break the game or if I can have a way to cheat the glitches out or take out one of the child's occult statuses with a quick cheat or cure, etc.)
Edited to add:
I found that a solution is getting rid of one of the statuses, if any glitches still occur.
"Only thing you can do is to either eat kelp twice to remove the mermaid or remove the spellcaster for them to be normal."