Iron Bars and glass panes are rendered incorrectly when connected

Iron bars and glass panes render their top and bottom faces when connected.

Steps to Reproduce:

• Summon a wall of iron bars and glass panes.

• /fill ~1 ~ ~-1 ~1 ~2 ~-5 minecraft:glass_pane
  /fill ~1 ~ ~1 ~1 ~2 ~5 minecraft:iron_bars

• Look at both the iron bars and glass panes closely.

Observed Behavior
Iron bars and glass panes render their top and bottom faces when connected.

Expected Behavior:
Iron bars and glass panes should not render their top and bottom faces when connected.