I have made modular walls and I have also made Light Maps for the modular set. Every time I build the lighting i have these problems where each individual piece does not blend well at all in shaded areas:
I have tried upping the resolutions of the lightmaps of all pieces and still nothing.
Also the ground and roof are the same model and lightmap and the ground shadows are good and the roof looks bad. not sure why.
When I started working with UE4, I encountered the same problem, the answer I was given was something along the line of “different pieces will be rendered by a different cpu core, and there will always be different results”. I don’t know the science behind this statement so please take it with a grain of salt, I accepted the answer and dropped that kind of modular assets for ArchViz for good (it was roughly 2 years ago, I moved on).
You can tweak the settings for absurd quality and you might be able to fix it or get really really close, but as far as I know, most artists create artificial seams to avoid this issue (imagine sci-fi corridor with something in between each wall tile). I don’t think this is the way to go at all.