Motion Vector problem - object particles

Using Fusion Particles and Replicate3D I am getting quite unexpected results for Motion Vector - in short it's totally random, changes every frame - can anyone confirm/deny - I am using Fusion 7 Free.
Scene is nothing really complicated, just some stones rotating and moving without any forces/turbulence etc - just plain movement.
Scene is nothing really complicated, just some stones rotating and moving without any forces/turbulence etc - just plain movement.