My thinking is that this happens because these vertices and edges in the borders share the same location, but are not merged, so they cast some sort of shadow or ambient occlusion to the adjacent module.
I imported the same building as an Static Mesh, with all the modules and vertices merged as a single object, and the Light Build works perfectly, no artifacts:
If you are going the route of using the build lighting system, you will have issues like this unless you modify the UVs to properly align and set the resolution higher per object.
The best solution is to make a mesh out of all objects, which can be done in the editor.
Increasing the Lightmap Resolution of the modules does improve the look of it, but I am afraid that the lightmap texture starts to get too big, specially if the building uses many copies of the module, and if there are many copies of the same building in the level. So things can get out of hand quickly.
I will probably go with increasing the Lightmap Resolution, but I am still curious if there is a Blueprint node that can merge the vertices after the desired height of the building has been established, not only to fix the light artifacts, but also to avoid the unnecessary processing of overlapped vertices.
