Not quite a bug, more of a documentation inconsistency:

The doc for "Make Parametric Grid Mesh" documents the following parameters:

U Expression, V Expression — The math expressions for the texture coordinate of each vertex. This defines how an image or other texture will be stretched across the mesh when the mesh is combined with a shader to create a 3D scene object. Typically, the values of these expressions should fall between 0 and 1, and should either always increase or always decrease, as U and V range from 0 to 1

But the node itself does not take these expression parameters. There are only expressions for X/Y/Z and they are expressed as functions of U and V. As a result, the UV texture coordinates cannot be manipulated independently, and the mesh created will always have a "flat" UV texture mapping, relative to the two parameters of the parametric surface.

I think the best way for this node to work, would be to accept mathematical expressions for all of X, Y, Z, U, V in terms of two other parameter variables (perhaps p and q?). This would make it possible to create arbitrary UV texture mappings independent of the mesh's shape in XYZ space.