Wamuu

Hey, so I've been playing on the new realm for a while, and I noticed that mobs that are summoned by other mobs, for example, the mechanical golems in deadmines, and the imps of warlock mobs etc, don't despawn after you kill the mob that summoned them. Is this a bug? Or was it like this in vanilla, because when I played on Nostalrius/Old Elysium they used to despawn.