Essence mob bugged

As mention in the title, essence mob is bugged and does not drop the essences when killed. not sure how it is done or interact for this bug to happen.

Attached a link to show the bug happening.

https://www.youtube.com/watch?v=C2Yt9HG58Uw

as seen in the video, i have the Sun Priest MTX enabled, which shows that the rare mob, in this video is the essence has been killed but no essences were dropped.