What's up with MOSFETs?
Do the same simulation on a vastlty different time scale, like 1000 times slower. So change the us (micro seconds) to ms (milli seconds) and run the simulation again.

Note how in the first plot the red trace is going down but before it reaches zero you switch the NMOS on again. It has no time to reach zero !

There is a large capacitor present between gate and drain and combined with the 12 k drain resistor that is a large time constant. Larger than the 1us you're allowing it. So slow things down and see what happens.

When you get the curve you expect, lower the value of the drain resistor and note how the speed increases again. At 1 us you probably need 120 ohms or so, not 12 k ohm).