I was exploring some strange artifacts in a render of a hair prop which appeared to show sharp corners and straight edge lines despite smoothing and one level of subdivision being applied. I wondered if the subdivision was somehow affecting the interpolation of the UV mapping inaccurately, so I decided to add the Cycles Wireframe node to see what the subdivided vertices were doing.

Lo, and behold, that node appears to triangulate the mesh in a way that neither the original mesh or subdivision shows.

The original render shows some wavyness in the striations which I can't attribute to the texture map, since it's just a lot of vertical, straight lines to my eye. Maybe there's something strange happening with the UV interpolation for subdivision?

Here's the Wireframe preview (the turquoise is due to the ambient colour which is where the wireframe node plugs in.

All of the subdivided facets are still quads, not triangles.

And here's the wireframe area render over the original:

All those diagonals have been (arbitrarily) generated by the wireframe node, since they don't represent actual facet edges in the mesh (whether subdivided or not). If they are actually arbitrary, even if just a product of the wireframe algorithm, surely there's a point where the algorithm decides to add one or the other diagonal to a quad facet. It would be really useful if it just didn't add a virtual edge at all there.

Has anyone else seen or remarked on this?

Yes. Can confirm similar experience.

Having trawled the Blender forums, a great number of Blender folks had requested exactly the same thing. Fault reports received feature, not bug, responses. Apparently, cycles triangulates everything during the render process, so by the time the wireframe node gets to act, the mesh it sees is only triangles, so it's stuck with that.

Workarounds appeared to be to use a reset UV map where every facet was an island (whatever that means, possibly that every single texture edge gets duplicated so shading groups aren't regarded as contiguous). It might just be a matter of using the rendered UV map as a mask, since the wireframe node doesn't do what anyone expected it to.