That's not buggy; it's not the same mechanic as it was in RTW. Back in Rome, you only controlled the square when you had troops in it and there were no enemy in the square at all. Even one enemy trooper in the square and you didn't control it. In this game, it's based on the number of troops on both sides in the square. I'm not sure of the exact number, but generally if you outnumber them by 2-3:1 you'll have control of the square even if their entire remaining garrison is standing there.