If someone sends you a friend request in a game, you get two notifications. One from the player list, and one via an on-screen popup.
If you accept the one in the player list, then decline the one from the popup, the person you just added is then removed.
I understand why this happens, so a potential fix is to remove the popup (if it exists) when a player accepts or declines via the leaderboard.
There, posted it Buddy. 
