What are Seasons? Where did my Medals go?
Medals over 4600 at the end of each Season are reset. This occurs for every player over the season amount to provide players a fresh start and competition on the Leaderboards.
You can view information about this in-game. Tap the Leaderboard icon on the home screen and then the "i" button next to the Previous Season. This will let you know what the Medal count will be for the current Season.
You can also view the remaining time of a Season by opening the Leaderboards.