Essence Monster Didn't Drop Essence

This happened to me in Valley of the Titans (Normal).

It was Walking Goliath (the big skeletons) with Essence of Body.

It seems like it was spawned as a corpse, as these mobs can do in this area, so when I released it nothing happened and I had to pull a Lich over to the body for it to resurrect it. Then it was resurrected (rare mob name: ROTLAUGH) with the Essence of Body mod on it, but after killing it, it dropped nothing.
I pulled a second Lich over and killed ROTLAUGH 2 additional times, still no drops.
I did not have the opportunity to Freeze/shatter it or corpse explode to see if that changed anything and "killed" it for real or something.