Like the season rewards, players are eligible for the Leaderboard Alliance reward provided that they stayed in the same alliance up until the cool down period of the event has ended and the rewards have been distributed to all players. Please remember that leaving your alliance when the rewards haven't been distributed yet will disqualify you from getting them.

Tip: To avoid missing out on Event rewards, wait for the prompt in the game that you got them, accept the reward, and visit the Treasure room to check if they are there. Once you see the reward, you can go ahead and switch Alliances.