From the screenshot it looks like the linear analysis points have been set up incorrectly. The open loop output should be setup on the outport of the plant subsystem. Having the current set up, you are linearizing the controller. The resultant linear model for the controller, for a step response of the error signal would be a ramp as you see because of the integral property of the PID controller.
If you would like more information to see how linear analysis points work, please check out this documentation: