It should be happening due to the solver selection and sampling time issue. You need to discretize the system and make your calculations accordingly. Since I do not have library for fractional PID, this will be a trivial answer:
1-When the Bode is plotted for your plant, it can be seen that the sampling time must be selected at least 0.02 seconds. To make things sure and guaranteed, let's choose it 0.001 seconds.
2-Afterwards, go to your Simulink model, open Configuration Parameters->Solver and change your solver type to fixed step and your solver to ode5. Set your sampling time as 0.001.
After doing these things, run the model and observe the results. Let me know if it works.
