This particular rare mob spawns in Iron Pine Peak, within seconds of it spawning it rushes into a nearby foothold and dies, if for some reason the foothold isnt there (it always is, since Iron Pine Peak isn't the busiest of places). The rare mob will try and run across the bridge, which is where the invasion forces running over the bridge gets him.
Here is a video of him spawning and running to check the view, after that he runs over the bridge.
Here is a picture I made to show how it usually looks like, hope you guys understand what I mean.
So perhaps move the spawning location of the foothold, or increase/decrease spawnrate of mob/foothold?