Possible unintended loophole:
I want to attack a group of people and fight in guarded. So I find some nobody nearby and attack them, shift to guarded, stop attacking then flee. I then move quickly to the group I want to fight within ten seconds. I attack them and start the fight in guarded with no need to shift from offensive to guarded over three rounds.
Possible fix (if this is indeed a problem):
If you leave the room the lock to the last posture used is removed forcing you to start new fights in the appropriate posture.