I noticed that I couldn't see my line of redstone dust under the glass as shown below. After filling in regular blocks and entering spectator it was still not there from underneath. Rails (the closest thing to dust) are visible from underneath with both glass and spectator mode. So that means the dust just does not have a texture on the bottom.

I would assume it never even had one. Placing on glass and spectator mode are more recent than when redstone was added. So it would make sense not to have added one. Rails were added later than redstone, but don't have this issue. So either this is a really old bug or rails were fixed at some point but not dust.

I have also included some screenshots to help better explain.