MPC toolbox with preview for a QVM does not work?

I simulated a quarter-vehicle model (QVM) with an MPC toolbox (with and without preview/look ahead). I tried preview both with and without constraints on control signal and rate of control signal.

Without constraints preview reduces chassi acceleration quite a bit but with constraints preview does not improve chassi acceleration which I find strange. When I use preview the control signal does not look well prepared.

Preview means that the road disturbance is known in advance and thus able to prepare the control signal. Constraints on rate of control signal (can't rise too fast) means that with preview a bigger control signal can be achieved. And if there is no constraints preview should be less significant.