Hi mate welcome
I don't have the Forester myself. But have you tried locking the door's and unlocking them then start the car. Both cars I had gave you about 20 seconds to get in and start the car before the immobiliser kicks back in and I then have to press the button on the key again or the fob in my case. As I have buttons on the key and a fob but the key seems to set my alarm off :)