When the Apple Watch is not physically on the course or hole of the round, the distance display will show 999 yards. This is because the distance to the green is over 1000 yards and the watch cannot display that large of a value on screen. When testing a round at home or not at the course, you will see 999 on the watch and distance from the tee on the phone.
If you are seeing this distance when you are physically on the course and holes let the support team know at support@golfshot.com.