Weird seam in skeleton mesh after edit in blender

I want to optimize the skeleton mesh and cut off all unnecessary as in this video 3 Solutions For Removing/Hiding Parts Of A Combined Character Mesh In Unreal - YouTube
I removed everything using Blender except the arms, but now the seams are visible.

The seam disappears in unlit and appears in lighting only.

I figured it out. It was ‘comput normals’, now i use ‘import normals’ instead.