Select the cube and press H to hide it so you can see the result. Notice that it cannot cut a hole. The Boolean Difference works best on solid, closed shapes. If you subtract a cube from a flat plane, it cuts a clean hole because the plane is simple and Blender can figure out what to remove. But if you have just four walls with no floor and ceiling, the shape is open and non-manifold. Blender can't tell what is inside or outside. In that case, the boolean creates an inset or changes the face in unexpected ways instead of making a real hole.
You have to make it manifold. One way is to add thickness using Solidify Modifier:
