I did this same thing with UE4 and pre-Blender 4.0 version of Blender. And I had no seams!
Any idea what’s going on and how to fix it ?
The visibility of seams is dictated by the way the normals of the mesh interact with each other at the meshes edges.
If you want the seam to be less visible then the normals should match direction-wise.
Make a clone of the two terrain parts join them together and auto smooth them (in 4.1 smooth angle modifier). Then you can use this transfer method.
Or if blender has a paint normal direction tool somewhere either built in or as an add on then perhaps you can just paint the same value on both edges.
You can also select the edge verts and press alt+ n for the normals menu (in edit mode)
And then paste it on the other edge.
Smooth angle modifier in 4.1 can also auto smooth by angle. Seems to import fine into unreal.
Sure, makes sense (hence I do data transfer), but I tried to do copy/paste vector (per your example, but sadly vertex by vertex as it can’t do it for the whole selection) and while in Blender it looked seamless - in UE it has same nasty seam as with data transfer method.
As if UE’s FBX import ignores custom vertex normals on the vertices located on the seam.
P.S. If I import resulting FBX into any other version of Blender I have here (3.6, 4.0, 4.1) - I get no seam. So the custom vertex normals are in the FBX.
Do you have the normal set to import or calculate?
I tried Import Normals / MikkT space and Import Normals and Tangents - same difference.