Crustaceans monsters does not drop essence (mob disappear)

Hi,
As title says, Crustaceans mob type does not drop essence if killed too fast, so on map with this type of monster it's about 30% less essence drop. I reproduce this bug everytime with this mob type. I'm not sure if that's about mob, skill I use, too much damage or that they are essence monolith but from what I saw, after killing Crustaceans it does not spawn Rock spitter.

If you cannot reproduce, please let me know so I can give you more details.