Seekers have a bug when you rollback ingame to the last save, and the seekers are spawned there already: they wont spawn they aliens. It's the sameish bug as the Turtles used to have by not spawning their carried aliens with a rollback. Its annoying on some maps when you're trying to maximize the score and have to rollback 2x in order to get the points/kills of the seeker spawned mobs.

Onslaught is a map where this happens, among others, but I guess it's a flaw in the logic, and not in the map.