Wierd lightmap issue

I have simple night scene with house mesh, which is devided into 10+ small parts. But one of the parts (on the screenshot) never bakes right, always with the same light borders. UV is ok, I tried two ways to unwrap it in Blender but the same result, margin is also ok. I even tried to increase lightmap resolution on this mesh up to 1024 but nothing changed!
Why this is happening and how to fix it?