Bottom part of iron bar rendering black

The bottom part of an non-connected iron bar is rendering darker than the rest.

This only happens if it is placed on a solid block, so this might be caused by the iron bar being inside the other block. Here is an example where the iron bar doesn't have a block underneath it

This somehow only applies for the bottom part. The upper part is just fine.

I wont go into how to recreate this, it doesn't seem necessary.