If you look at a wall of glass panes from below, the bottom textures of all the glass panes are visible when they shouldn't be. Similarly, if you look at a wall of glass panes from above, the top textures of all the glass panes are visible.
If you look at a wall of glass panes from below, the bottom textures of all the glass panes are visible when they shouldn't be. Similarly, if you look at a wall of glass panes from above, the top textures of all the glass panes are visible.
Intentional, see MC-50349.