Something very interesting (to me) just happened while I was recording video for my promised video on fixing the mermaid scale problem.
I started a new game and added all four of the Isla Paradiso mermaids, Triton King, Mia Azul, Maya Ocean and Salty Seaworth, to my household. I had Mia Azul under the ocean so I could fix her scales while she was under water (this isn't necessary you can do it on land if you want to) and while she was swimming around with her scaly legs and no tail she was looking at coral and pink hearts floated up. I wondered why she was so happy with the coral when a new mermaid appeared and he had his tail. So he is game generated. It seems only the four Triton, Maya, Salty and Mia have the scale problem. I thought it was a problem with Isla Paradiso but now it seems it is just the four main IP mermaids and if you add them to an active household the game will generate a new mermaid and that mermaid will have scales and a tail. It is the default colour but it is scales and a tail.
https://s5.postimg.cc/79grmpu13/New_Mermaid001.jpg