Sometimes everyone that has started running gets the chest when the first or second person to complete the rush get across the finish line..
Sometimes you need to personally finish it twice before getting the reward..
Sometimes nobody receives the reward until the entire group finishes the event and you “end it early” (or in case of lack of runners/people managing to finish, timer runs out)..
Intended? It seems very random..
Atleast before the overhaul of Guild Missions everyone got their personal reward when they personally finished the rush, which made a lot more sense.. x.x