I suppose, because Luffy doesn't have full awakening mastery for now.
His heartbeat restarting looks more like just a animated version of the zoan awakening fast recovery. Presumably after a defeat an awakened zoan comes back but with a less health each time. Though there is still a limit to that and for Luffy his current limit is reliably 1 time.
His heartbeat restarting looks more like just a animated version of the zoan awakening fast recovery. Presumably after a defeat an awakened zoan comes back but with a less health each time. Though there is still a limit to that and for Luffy his current limit is reliably 1 time.