Essence imprisoned monster disappeared without being released

Bug Report #: 4,225,131,465
League: Mirage
Map: Lava Chamber

I entered the boss room, noticed an essence mob, got engaged by a pack behind it, decieded to clear the boss first. I moved forward and engaged the boss, after clearing the boss and returning to the essence mob it just wasn't there anymore. Upon rewatching a clip the essence mobs briefly became targetable and died (mostly off screen, inferring from two of the child mobs) when the map boss was around 3/4ths hp. I couldn't discern if the release was timed to a player or enemy action. I could not see any loot from the essence mobs.

Build: Kinetic Fusillade Totem Champion.