Why does UV orientation affect tangent space normals? When using a tangent space normal texture, the orientation of UV islands affects the normals even when using the Generated or Object Texture Coordinate. How can a UV map influence a texture that is NOT using any UV map ? How can tangent space itself be affected by the UV map ?

Here's the basic setup :


a plane split in 2 faces, each one being rotated differently in the UV map
a noise texture with generated mapping.


And here's how it looks :



Here's the file 

See how turning one face by 180° in the UV map removes the seam, while the noise texture is using a generated mapping.
The normal map node takes the default UV map, if one exists, to generate the tangent vectors. So while your noise texture uses generated UVs, your normal map node still uses your UV map. Pick Object Space in the Normal Map node instead to fix it.

Tangent space is affected by the UV map because the tangent and binormal vectors basically just represent the up and right directions on the UV map. So when you rotate a poly on the UV map, the tangent vector rotates with it.