Banners Turn Black When Placed Under Stairs

When putting banners under stairs so that the very top of the banner shows, the banner turns black. 

The banner returns to its normal color when a block is broken next to it, exposing the banner to the light. (As shown in the picture)

Steps to Reproduce:
1. Dig a 1 block deep hole 

2. Place a banner in the hole

3. Place stairs over the top