The Bug:

Bone meal cannot be applied to moss if there is a block above it.

Steps to Reproduce:

• Build the setup shown in the attachment below. setup.png
• Attempt to apply bone meal to the moss block on the left side.
• →  Notice how the bone meal is used and the moss spreads.
• Attempt to apply bone meal to the moss block on the right side.
• →  Notice how you are unable to do so, therefore indicating that moss requires air above it in order to be bone mealed.

Information:

This could be intentional.