The top and bottom parts of the iron bars block use two textures each. One for the top, and one for the bottom, with a total of four textures. All four textures are obtained from the middle of iron_bars.png.

The bug

Both bottom textures are flipped compared to the top. Both top textures match the pattern shown in the middle of iron_bars.png, but the bottom textures are rotated 180° by default, making the pixels for the top and bottom not line up.

A pack is attached that rotates them 180°, which fixes the issue.