I’ve figured it out.
You can all relax, because I’ve figured it out. Why that last shambling horror always spawns moments before the LK runs away to do his “ooh, chilly” phase transition. Why the valks ALWAYS pick a healer when infest is coming. You can blame every wipe on this.

Let’s just take a closer look at that, shall we?

So there we have it.
It’s all Dy’s fault.