[Fixed in 1.13] Some pixels in the middle of the iron bars turn invisible when surrounded

Steps to reproduce:

1. Place iron bars on the ground;
2. Surround it with more iron bars in all four sides.

Observed results:
The middle bit turns invisible, resulting in a big gap that shouldn't be there.

Expected results:
The middle bit stays the same.

 

Extra:
This is probably caused by the feature that does the exact same thing to glass panes (which is intended behavior), however, it shouldn't do that since it isn't a 3D model, but actually flat textures.