Rares trapped by essence scarab in Betrayal's Syndicate Laboratory don't drop essences

Using Essence Scarab of Calcification, every rare in Betrayal's Syndicate Laboratory (Research) is trapped in essence. Releasing the rare and killing them doesn't drop any essence, however (!) it does give the last boss the essence from the rare slain, despite the rare not dropping any.

I've tested this for at least 20 runs, no rare give essence when released and slain, and the last boss is gaining the essence of the slain rare in the Betrayal's Syndicate Laboratory.

I don't think it's the expected result, either the rares in the Syndicate Laboratory shouldn't be trapped, thus considered non-natural inhabitants, or they should drop the essence they're trapped by.