observer and piston not working together right

when you make the below pictured circuit the observer sends a redstone single on activation by melon growth and activates piston. the piston then pushes and breaks the melon. when the piston retracts the observer is activated again, this makes the piston extend and retract forever without stopping. this is a bug i believe because i looked up on the wiki and it says that the observer can not detect the piston head this also happens when grass spreads to dirt.