Hi,
Normally i do a 15 minute warm out before starting an event/race. When the event is about to start i jump to the event without restarting Zwift.
Workout statistics for heart rate and power distribution for the main event/race is based on the 15 minute warm up - not the event itself.
See https://zwift.com/feed/484001380687551136 or any other of my Tour de Zwift events.
Best regards
Øyvind Odden