I use PC windows, I joined a game with Mean and a few others (Destroy observatory balloons) I didn't see any other players, so I checked the leaderboard and saw the practice mission exit button, my xp now says 100/100 now too, the game is also normal the first little bit then it basically turns into a practice mission I also don't know how to do images here.
Also edit: I joined another game and it didn't happen again, maybe it was a one-time thing.