Double tall grass created by bonemealing moss blocks just grows the bottom part if a block is above it. This one is hard to explain with words, so I'll just tell you how to see it.

How to reproduce:
1. Place a moss block, then place dirt in a 5x5 radius around the moss block.
2. Two blocks above the moss block and the dirt blocks, place stone.
3. Use bone meal on the moss block.
4. Look at the double tall grass.

What is expected:
No double tall grass grows due to a lack of space.
What really happens:
There is double tall grass, but only the bottom halves.