Blender to Unreal FBX export problem

Hello. I have run into strange problem, when I was trying to divide big mesh (a planet) into several smaller chunks.

When I prepare those chunks (for now I test with only two of them) in blender, they look fine:


(probably noone can see the seam between those chunks in this picture)

but once they are exported to FBX format and imported to Unreal, something bad happens, probably with normals:

For now I have no idea, what can be wrong here… I suspect Import Settings, but “Import normals” is checked.