Opening the serial port resets the Arduino. The Serial Monitor app doesn't allow you to type data and send it to the Arduino before the reset is complete. Your code DOES send data before the Arduino finishes resetting.
Try making your app be patient, and give the Arduino time to reset before you start sending it data. How long? Well, the best thing to do is have the Arduino send something that indicates that it is ready, before you start sending data. Then, the answer is "Until the Arduino says Ready!".