Getting leaderboard scores by "Anonymous Player" even though sign-in is required

In my recently released game, I’ve made it so that scores are only sent to the leaderboards if the player is authenticated. This triggers upon a “Game Over.”

However, there are scores popping up by “Anonymous Player” - at first I thought someone had named their account that way, but I’ve set the leaderboards to only display a user’s best score, and there are multiple Anonymous Players on the same leaderboard. You can check this here: Cuboid | Play on gd.games

Does anyone know how this is happening?