Happens all the time, more so when car is cool. Back out on a downhill slope hold brake, put in drive. It takes 1 to 3 seconds for drive to engage but nothing shows when it is engaged. You can however hear the ping of the engagement with window down. If you remove foot from brake and press gas the car will roll backwards 6-10 feet towards oncoming traffic. This occurs with or without the hill assist on. Subaru says this is normal nature for the car.