It would probably be best not to over modularize your level in this type of way, unless you have a supporting geometry that will hide the seams.
Lightmass can only adjust the settings so much before you’re fighting a losing battle in this sense.
Using your normal map and a plane I was able to to setup a few examples. The seams will not completely disappear without adjusting the static lighting scale, which I do not recommend unless you know what you’re doing.
For the examples below I adjust the following settings in World Settings > Lightmass
- Indirect Lighting Quality: 2
- Indirect Lighting Smoothness: 0.6
These examples use a total of 9 meshes. Three planes for the each wall and the ceiling.
Example one using two lights with radius of 650 and intensity of 5000
This next one is the same thing as above minus the light closest the camera.
In this you can see the seams. This is due to modularity in the mesh. Typically using masking geometry or adjusting lightmass settings can help reduce this. Not always though. Lowering the settings too much will cause other artifacts to appear. This is not a bug, just working within the system to get something that works.
In this last image, I’ve scaled the width of the plane down to half size. You can see the moduarity is exacerbated with the seams.
Without the normal map
The seams are created by Indirect lighting (bounce). In the first image there is enough light in the scene that any bounce blends in and you do not see the seams. When trying to troubleshoot a lot of issues like this it’s sometimes best to start eliminating other factors that may be the cause. For instance, using just a simple base color material you can eliminate other factors like metallic, roughness, or normal values that may exist. If you don’t see the issue with the base, start adjusting or adding in textures until the problem occurs. This way you can start to see where the issue is coming into play.
Some of the problem also appears to be that maybe your diffuse isn’t able to be tiled which looks like part of the seams issue.