That kind of workflow is not a good idea
A few things–
First, the way the lighting works, it processes each mesh on a separate thread which means that indirect lighting will be slightly different on each part. When you construct a flat surface out of several meshes, then you’ll see the seams between them
The next issue is that it’s very bad for performance–each mesh is a draw call, so by making one object out of many you are creating a bunch of draw calls which will impact performance. The main reason for a modular workflow is to save memory, but since those meshes are extremely simple, you aren’t really saving much memory, it would be better to make a unique mesh that’s the exact size of the surface you want than to construct it out of separate pieces.
Another thing–don’t have surfaces around the edges of those meshes, they won’t be visible or contribute in any way, they can cause seams to appear as well so it’s better to just delete them. It does help with lighting for an interior if you actually have an exterior surface but you don’t want surfaces on top of each other.