I'm on Xbox and this happens to me too. After getting Realm of Magic about a month ago I had my Sims eat Mermadic Kelp twice to stop being mermaids so they could be Spellcasters.
I had my female Sim enter the water but my male Sim (one of the premades in Sulani who isn't a mermaid but I made into a mermaid at the same time I made my female Sim one) didn't because they were in the Realm of Magic when I decided to make him a Spellcaster too.
Only my female Sim gets the mermaid animations before returning to normal but the male Sim doesn't.
The Kiss in Water interaction is also affected by this glitch since when she does it, her legs do the tail movement also but return to normal once the interaction is over.
Hopefully this gets fixed.
EDIT: Just read through the thread and did what @crinrict suggested. They're in Mt. Komorebi right now eating (on the slopes) so when they're done I'll see if the cheat worked. They're still able to go home though since they're visiting the slopes from their house.
EDIT 2: Yup, it worked! 😊 If you're reading this, thanks for posting that cheat to remove the issue, Crin! 😃
