I have the same issue here, but this time, my sims never was a mermaid, but exchanges a kiss with one pre existing mermaids in the game, and since then, he litteraly turn is leg into difformity legs in bath, and he does the anim underwater to change into mermaid, but he still just have is legs. By the way, he was never a mermaid, he was born spellcaster and still have this issue. Other sims that was once mermaid and that I have change back to normal sims trought the kelp fruit, are now having that issue as well.
Did you manage to fix the issue? It definitely worked for me. Mine was a mermaid to begin with and removing the trait helped but since yours wasn’t one before I wouldn’t know much about it. BUT definitely try to do what Crinrict mentioned in the previous post!
