Keysight's Advanced Design System users, what's wrong with this simulation - II
Having looked at your results we see all the terminal voltages and the channel current, I suspect there is a fault in the model (or gmin in the simulator is set to an extreme value). To see if it is resistive (or a gmin) effect but the rest of the model is working, try ramping the input voltage all the way to 24V. If the output starts to ramp when vin gets to about 15V, then check settings. Try a different transistor model.