Lightmap seams, on modular objects

I’ve got the problems with lighting, on “modular” meshes - there are visible “seams”.
Each separate “piece”, got it’s own “brightness”, depending on the distance from the light.

Is there a way to fix that somehow?
Can somebody explain - why this is happening? It looks like there are some additional lighting calculation, from the mesh pivot point.

Lightmap resolution for walls: 64, floors: 128
Light on the wall is stationary, with dynamic shadows.