1.Expected Behavior

The moss shoul be able to spread in a 3*3 area up to a 7*7 square area. Recatangles are possible.

2.The bug

When you bonemeal moss you are unable to get the full 7*7 area thus, the corner blocks are never filled. (I automated a setup and filled twice a dispenser with bone meal (18 stacks = 1000+ attempts))

3.How to recreate: 

make a 7*7 stone platform.

In the middle, put a moss block.

Under it, put a dispenser with bone meal inside.

put a 7*7 of downward facing pistons so they can break the moss carpet and things like that.

activate al the piston with simple redstone and the dispenser as well.

hook that up to a simple observer clock.

after the nine stacks of bone meal (I did it a few times to be sure) you will see that the corner blocks are still stone