@Jomilej I agree with @jpkarlsen and @gooochyeee that this is by design and not a bug. The mermaids having tails in bathtubs was probably just a cutesy thing reminiscent of the mermaid in the 80s movie "Splash". Since sims don't swim in the hot springs, it might be a little awkward to animate the change. Would you expect the mermaid to change before they sit in the hot spring and then change again before getting out? Whatever the case, I have a feeling this probably won't change but I suppose it doesn't hurt to ask for it.
