why linearisation of a closed-loop systems results in an infinite step response?

Dear friends!

Please, take a look at the system below

with open loop inputs and outputs configered this way, the following is the step response that i get:|

the question is: how come? My PID is tuned as follows: Kp = 10, Ki = 50, Kd=10. These settings are arbitrary, simply to check stability margins and how coefficients influence it. But the system judging by this response is clearly unstable, while position scope shows othewise. Please, if you have any ideas, share them with me!