This is close bug. What I see happening is that when you build you remove the spawns from the nearby area. I even think that this nearby area is quite big. This combined with that hostiles despawn.
SO you can conquer a node by node by just putting an electric pole close by and walk away.
When you come back the monsters are gone.
I can understand why this is done. You don't want monsters to spawn in the base. But the effect is also not wanted
I would recommend change the range of this effect to the amount of buildings blocks placed and exclude electric poles and belts, and belt supports from this.