I don't understand why with a custom workout, you have to manually stop the recording at the end of it ,contrary to the Garmin Coach workout which auto stop at the end.
The custom workouts are designed to give you, the user, more control, so the auto-stop is not built in. Garmin Coach workouts are not created by the user, so they have this "auto-stop" feature built in, with an additional option for the user to resume the activity after the auto-stop.