OK, I was able to do it 3 different ways. The way you describe worked. I used a boss spawner so that when the cahllenge was killed, the cutt throats camp spawned.
Last I tried without having the auto-spawn checkbox on and setting them to de-spawn when the scenario started, and then spawning them when needed again. All 3 worked.
Try downloading this file and trying it.
Lay down the challenge.r2
I think you should just be able to load that scenario using the client. It isn't pretty or anything.