IME, when this happens it means that some enemies in that wave have either gotten lost (like baby animals wandering away), or that some have spawned under the mesh and can’t escape.
Given that you are near those rusty-red cliffs though, I can tell you that up on the Mounds/Highlands side (i.e. Riversend) purges will definitely fail for this reason because there are two separate terrain meshes that come together – the one with the rust-colored mountains, and the normal ground mesh.
If you or a friend have admin rights on the server you’re playing on, try activating Ghost mode to fly under the map to check for any such stuck enemies. If you don’t, then load up the game in single-player, activate admin mode + Ghost, teleport to that spot and inspect under the map yourself to see if there are any potential problem areas.
In some instances, you may be able to place a few blocks into a shallow recess to prevent purge enemies from spawning in there, but often as not I’ve simply given up on such sites and moved elsewhere because it’s too much of a hassle to deal with such repeated disappointment.