Lightmap look ok on hard edge, not on soft edge with normalmap


I have a problem with my meshes; They have soft edge with a normal map
it appears strange on the bottom of the mesh, and the shadows behind the mesh are not great.

I noticed that when the mesh has hard edges, there is no problem with it, and all become ok. I specify than the mesh with hard edge has 64 resolution of lightmap, the one with soft edge has 512.

I already tried increasing resolution of my mesh, snapped on grid (3dsmax), increase the bound, played with lightmass settings, and nothing work …

Someone have an idea?