Warden not spawning, only spawning on ancient city

So i was just making a Warden farm and i notice that the warden dosen't spawn even if it was activated it, it only spawn in ancient city and if you try doing it on surface or underground it dosen't spawn the warden, but when you go over to an ancient city it spawns the warden. (video here: https://youtu.be/MgFQMNkf-AA )