Cobalt Scar Mobs Falling Under The World when Underwater
 

When fear kiting underwater the mobs can randomly fall under the world. As a necro the pet will follow them under the world and most of the time come back if kills the mob, but not always.

They’re not falling through the world, they’re pathing into the blueish stone layer.
Definetly a pathing bug that should be fixed, but try fight them over at the sand area (bottom level) near the wreck as a workaround, they behaved more nicely there at least for me )