I've been having this problem with this modular floor system and lightmass for a very long time. I've simplified the scene significantly for the sake of this thread.
These floor tiles are perfectly square static meshes, 192x192 UU. They have four subdivisions both horizontally, and vertically.
It has a perfectly square UV, with no seams; it even has a separate lightmap UV with no seams. The lightmap UV resolution is 64. The scene is also rendered with production quality lighting.
In the image I have attached, each tile is scaled to .99 percent, so they are not contacting eachother, in addition it is a screencap with detail lighting enabled, so you aren't seeing the diffuse.
As you can see, there are two problems: Tiles further away from the light are darkened entirely - instead of having a smooth gradient from light to dark, the further away the tile is. the entire tile is shaded darker - almost like each tile is a single pixel, in a rough gradient.
The second, and more pressing problem is the rotation issue... some of these tiles are rotated at 90, 180, and 270 degrees - causing the tiles to have completely different lighting, based on where the camera angle, resulting in the terrible patchwork appearance you see.
The regular material for the tile has diffuse, specular, emissive, and normal maps. At first I thought the problem was being caused by rotating the normal map - but as I peeled away the layers, and simplified the material and mesh, I found this problem to exist, even when there is a simple diffuse alone.
We are using modular tiles, which must be rotated occasionally. (For instance, a floor that has a certain detail across one edge.) Even using a solid bsp doesn't seem to fix the issue.