Its not a bug that was recently introduced its been here since beta. I believe it was implemented into the game for new players. Since new players usually build up their armies until they have max pop and just throw their armies into each other right clicking a single unit and all your army trying to kill that one guy would mean really awkward looking fights and losing your entire army. I guess they put this into the game without thinking about the community and how this would effect game play.
I already made a post about it here