Tryed to make safe place for the crops but two slabs that had 1 block hight from another blocked light.

What I expected to happen was...:
Light go normally between the slabs and let the crops live safely.

What actually happened was...:
The slabs magically blocked the light even that there was one block height between them.

Steps to Reproduce:
1. Its very hard to explain it so is it ok that i have an video of the happening? Just email me and i will sent it to you because its too big to attach to this report