Version: 1.56 RC
Groups with a "Guard" waypoint will not move to "Guarded by" triggers of the same side when placed in the 3D editor. AI groups will move to their guard waypoint and no further. Note that it doesn't matter how close the waypoint is to the trigger, nor if it is within the radius of the trigger.
Furthermore, guard units will not move to engage enemy elements known to their side. They will only engage units that the group's leader knows about.
This only effects the 3D editor. "Guard" behaves properly in the 2D Editor.