Ideas
Same! I have noticed when I change this particular family of vampires into something else, they still have traces of their vampire problems.
For example, I aged down one vampire into a child, (which is something I've done several times because I really play with same 5 sims in different saves but just change a few things about them) and there is usually no problem, but now they still gets the Thirst moodlet! And not just them. 2 more members of the family got switched to spellcasters-but they still have their weaknesses like thirst moodlet, not being able to digest food.
I have tried to give all afflicted sims plasma, but teen-elder sims that aren't categorized as vampires can't drink plasma without getting sick. And the kid vampire can't drink plasma or plasma products at all.
From what I've noticed, it's only this family affected with this. I could be wrong. I turned another vampire into a regular sim once. But he was a background character, so I didn't play with him long enough to get "thirst", so I don't know if he has the same afflictions. He, and the others are still able to go out in the sun.