This is after an hour of being AFK in my base, it was empty when I left:

You can see the config for Primitive Mobs here, and the config for MultiMob here. These are the most recent changes I've made and even though there has been a server restart since then, they're just appearing out of nowhere.

For context, my base is a Plains biome that I've littered with trees but that is also fairly well lit. From my understanding they can spawn on any lit block, so perhaps that is increasing their spawn change artificially? This is what it looks like on the map, and this is the minimap: every single green dot is a travelling merchant. I've found them hiding in the craziest of places.