There are two different monetary policies. The green monetary policy gives you the green AD curve, and the red monetary policy gives you the red AD curve. Both monetary policies give you exactly the same equilibrium P* and Y*. (You can interpret P* as the price level, or as the inflation rate, as you wish.) Which monetary policy is best?
If you are an old economist, like me, you will probably say the green monetary policy is better. Because the red monetary policy leads to an unstable equilibrium.