Essence Mobs Vanishing

I checked known issues and looked for anything posted about this.
For some reason when fighting essence mobs in the Leyline map they just vanish. Typically occurs when they are in culling range. Not all mobs do it, but I think it's just the crabs. No loot, no nothing, just gone. I've tried a few other maps and had no issues. I can easily replicate this if required.

EDIT:
Happened again in a different map and with the "Shielded Crabs" mob type.