Feedback [bug?][ARFoundation] planes drift in the Y axis by +/- 2cm

I tested this using the Unity ARKit Plugin and I see the exact same behavior between ARFoundation and UnityARKit. However, it is more obvious in the ARFoundation Samples due to a few factors:

1. In the ARFoundation SimpleAR scene, the cube is placed at a fixed position directly on the plane. This makes it quite obvious if the plane moves up or down as it covers or reveals the cube.
   
2. Not sure which sample scenes you were comparing, but the UnityARKit scene UnityARBallz, for instance, places objects 3 centimeters above the plane.
3. The balls in UnityARBallz are full rigid bodies, so they move up and down with the plane, making it difficult to detect minor movements.
4. Even if you remove the rigid body component, the plane in UnityARBallz has a nearly transparent material, which makes it difficult to see when it intersects the placed object.

When I removed the rigid body component from the ball prefab, placed it directly on the plane rather than 3 centimeters above it, and changed the plane's material to something more opaque, the results are the same as in the SimpleAR scene from ARFoundation.

In addition, I would be quite surprised to find this sort of behavioral difference between the two SDKs. Neither UnityARKit nor ARFoundation manipulate the plane's position -- it is whatever ARKit tells us it is. Poor frame rate could affect tracking or plane detection fidelity, but both SDKs easily achieve a solid 60 fps.

If this doesn't sound right to you, or you have more details on comparison you performed, please let me know.