I -think- I had the same phenomenon during "Eztli-Tenoch's Right Hand", but I'm definitely seeing it in "The Way Out". Monsters spawn and go -away- from the game area, straight left off the screen, with no walking animation. Several waves later they will march back towards the right and into the game area. So far I've seen it in normal and extreme difficulty, and the game speed does not seem to have an impact. I don't recall this from playing earlier versions and I assume it's not intentional?