The Post Game Carnage report doesn’t work for matchmade social games if you are not the leader of the game session.
It works when the match ends, but after the session leader leaves the post-game lobby the rest of the people in the session other than the leader don’t have the option to open the post-game carnage report. This isn’t an issue for competitive games, only for matchmade social games.
This is on PC, not sure if it is an issue on xbox.