Yes, they were mermaids born from normal gameplay between a human sim and a premade mermaid sim in the game.
I'm having the same issue with a lot of my Sims... as well as many other bugs that seem to have started happening after downloading the Magic Realm expansion pack. My main Sim is a mersim and has many children that are also mersims; some adults, some teens. 3 different things have occurred now. First, my Sim wouldn't change out of her tail and was "walking" around clumsily on land while still sporting her mer-tail. Then her and a couple of her offspring were either swimming around like they had a tail but were sporting legs or they wouldn't even acknowledge the fact that they were mer-sims and were just swimming around like normal people. Now, my main Sim is swimming around with only the upper half of her body... sometimes even swimming around through the house and on land until I shift click and reset Sim.
Like I said, this is only one of many annoying bugs I have been experiencing since Magic Realm was downloaded. I have never used Mods on the games.
Same here, after making my teen a mermaid by eating kelp he had no visible tail. Creating a tail in CAS fixed the problem permanently.
