I just tested importing an alembic skeletal mesh (exported from blender in my case) and it looks like the actual normals are wrong, the alembic mesh has a hard edge at the UV seam. So this has nothing to do with the normal map or tangent space (You can turn tangent space normals off in the material and plug nothing into the normal input and you’ll still end up with a hard edge). The only workaround possible here would be to treat all your UV seams as hard edges in your DCC app and bake your normal maps with those hard edges (probably brakes when animated though and it should be fixed on engine side anyway).