> ARB_fragment_program_shadow may leave it undefined, but the GL spec ...

Which spec exactly? GL specifications only cover GLSL shaders.
ARB_fragment_program is separate from that and has its own rules. The extension
specification clearly states that behaviour is undefined.

Besides, OpenGL contradicts itself a little bit: in GL 4.4 spec, 15.2.1 it
states that behaviour is undefined for this case, too. It makes much more sense
anyway, shadow sampling is wildly different from sampling a texture normally in
terms of what the sampling result look like.