Editing a sign side with no text on it while holding a glow inc sac grants the Glow and Behold advancement, but does not make the side glow.

If you interact with a sign, while looking at one of its sides which does not have text on it, while also holding a glow inc sac, the Glow and Behold advancement is granted, but that side of the sign is not made to glow because the text editing menu is opened instead.