Without changing too much, set the GravityDirection of Signal Action 1 to 230.
Why? The Vertical Stacker is attaching parts in the stack in which the top part of stack is parent/root of entire stack. So when the robot places the stack, it assumes the parent is the part at very bottom. As a result, the gravity direction of 1mm would not work because no container/component would be detected for top of stack. Hence, the entire stack is released to 3D world and not attached to table. Therefore, using a greater gravity direction is necessary for the stack parent to collide or detect container/component to attach itself to in 3D world.