Step response of an undamped 1DOF model in Simulink is still damped

I tried to simulate a 1DOF mass spring model with asymmetrical spring with k=10^9 N/m for x>0 and k=5*10^8 N/m for x<0 (model attached with name "KnickFeder"). If step excitation is applied with initial condition x=0, without damping, the amplitude shall stay unchanged. However in the simulation result below is apparently damped.

I tried a simplified model with linear spring (model attached with name "LinearFeder"). The result is still damped, albeit to a less extent.

If I build the model with SimScape, the result is perfectly undamped.

The parameters of simulation are the same (0.1 ms time step, 2 s duration). Yet why the results are different?

[rubindan]

Your system frequency is

You simulation is running at 10^4 Hz, and that is simply not enough. Decrease simulation sample-time and you should get correct reasults.

[DFfd]

The system frequency is