I've run into some minor compatibility issues with the minecraft essential mod.
https://www.curseforge.com/minecraft/mc-mods/essential-mod

If the settings shown below are disabled (Allowing for essential's cosmetics to show) the mermaid tail does not render. Unfortunately I do not have screenshots of the tail not rendering though.

Its a small issue and easily circumvented, but the odd thing is that when I load up the world again I have to navigate back to this page to restore the tail each time as it becomes invisible again. I don't personally use the cosmetics anyway but I thought you might want to know about this bug.

Whoops! It appears the incompatibility is actually with Cosmetic Armor Reworked:
https://www.curseforge.com/minecraft/mc-mods/cosmetic-armor-reworked
Whenever the chest cosmetic slot is toggled, the tail disappears: