Unexpected behaviour with fresnel on procedural mesh

I’ve created a procedural cylinder shape, and applied a material to it with a fresnel, but it’s producing a strange effect. Attached is an image comparing a standard static mesh cylinder next to the procedural one, both with the same material applied.

When I am looking down on the procedural mesh, I get the turquoise colour, and when I am looking up at it I get the pink colour. As you can see from the standard cylinder, and as per the standard fresnel effect, it should go from turquoise to pink depending on the viewing angle of the pixel from the camera.

What is going on here? Is this something to do with normals? Tangents?