It's hard to explain, but in the screenshots in the attachments, the shading turns black for certain sides of the model depending on the blocks next to it. So, if the block has another block on the right of it, all sides of the model facing right will be black. Along with the screenshots, I will also attach the resource pack so you can test it out yourself.
Edit: I have smooth lighting off and graphics set to fast.
Confirmed. Added another screenshot