Players are eligible to receive Season/ Event rewards provided that they stayed in the same alliance until the season ends or after the event cool-down period has ended and the rewards are distributed. Please take note that leaving your alliance when the rewards haven't been awarded yet will disqualify you from getting them. 

Tip: To avoid missing out on the season/ event reward, 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.