I thought it was due to some mod until I realized that even when the game runs without mods, this glitch occurs. I haven't seen any fairy sims born in the game or fairy NPCs have this problem, but if you create fairy sims in CAS or even through Mother Nature, this problem eventually occurs.
For example, I have this family; the mother became a fairy through Mother Nature, the older daughter (born in the game) I transformed into a fairy through CAS, and the younger daughter (also born in the game) was born as a fairy. Only the younger daughter has never presented any problem. The detail is that even if the glitch doesn't occur immediately, when the sim becomes a teenager or a later age, the problem appears (except for those born in the game or NPCs, as mentioned).
The only solution so far is removing the fairy form via CAS. If the sim becomes a fairy again, the problem may or may not reappear. It's quite random.🧐