Aren't they supposed to do this?
Usually, they do this if everytime they the game tries to grow a tree, certain conditions are not met. Are there any blocks near the sapling.
Accordding to the wiki, it's supposed to break in 2.55 seconds. But in MCPE, it breaks in less than a second
Still an issue as of MCPE 0.15.0.
It once happened to me in the Overworld.
Still works in 0.15.2