Hey jpair!
Upon initialization of the game, the user automatically receives a unique identifier from the server, which is used for identification. This is then used to identify the player's entry among other entries in the leaderboard. I have just released a new update which should fix the issue of unique identifiers getting lost, however this behavior may reoccur if your leaderboard exceeds the limit of entries that it has (1000 for classic leaderboards and 100,000 for advanced leaderboards) as entries outside the limit get removed completely.