It's not correct because the mermaids don't transform and have the wrong need bar. But the trait mismatch is consistent, and my best guess at the problem.
Now then, I am aware there is a separate trait (trait_OccultMermaid_Discovered) that indicates if a mermaid has been outed. Doesn't matter. In my own testing, that fact was entirely irrelevant to the issue. Nalani has the correct trait in a fresh game even before I interact with her at all. She won't transform until outed, but once outed she behaves normally. This is not how bugged mermaids behave.
When I first encountered a bugged mermaid, I asked her to join my Sim's family with the Friendly command (and upon doing so, she HAD the "outed" trait - which is distinct from the two listed above - but still would not transform. Since it came up.) In my follow-up test with the mods disabled, I added a spawned mermaid to the family via Manage Worlds. Both times they remained bugged. No cheats.
MORE INFO: Maybe trait_OccultMermaid is sometimes there and sometimes not? Or it gets replaced by trait_OccultMermaid_Discovered? I just started a new game to get screenshots and used the trait checker on the moment I saw Kalamainu'u. She had both trait_OccultMermaid and trait_OccultMermaid_YTAE (she's pregenerated; does not negate the bug.) LATER: Yes, Nalani and Ukupanipo also have both traits in a fresh game. So the important thing appears to be whether or not they have trait_OccultMermaid_YTAE. Not having that seems to be the bug.
