Mirrored uvs show seam even with neutral normal map

Hmm, I would check your geometry. If you’re saying it does this even with a plain normal map, then this has to be a problem with the mesh. Make sure that is indeed only a single vertex and not two very close together, and make sure it is consistent on both ends. You could also double-check how the tri conversion was handled when it was imported by inspecting the wireframe of your mesh in Unreal Engine and make sure everything looks good there.

All that to say, this looks like a mesh issue, not a material issue.

Good luck!