Why is this a bug that needs to be fixed? Because it makes stealth based missions pretty much moot. It is no longer a question of situation whether you can risk taking out a guy or not, you will have to ask yourself, is he grouped or not? Because if he is grouped, it doesn't matter where his buddies are, they will know. If they spread out way outside hearing range, if they are guarding a point and sent out a guy to check something, you cannot take out that guy because they will immediately know that the guy was shot.
The argument that you use scripting to alter the group's behavior is irrelevant. If you want naturally behaving AI, you will have to resort to scripting. Take BIS_fnc_taskDefend. According to its description, AI will man static defenses as well as send out patrols. Patrols will be untouchable since you cannot dispatch them - they are grouped and therefore the rest of the group will immediately know.
Another case, setting a group's formation speed to full says the individual members will not wait for the rest of the group. A DISMISS waypoint can have group members wonder off into the distance. Still, all of the group will know about any dead member, because they magically share the information.