Negative Voltage In Capacitor When + Side Is Connected To -
Run your simulation at maximum speed with 75% current speed. Observe the capacitor voltage. Your capacitor is so large and your 10K resistor is so large that after the capacitor charges up, it takes a long time for the capacitor to discharge and while it's discharging the current that if you run at slow speed you are perpetually stuck in a transient state that looks like steady state.
Observe the current direction through the 10K resistor before and after switching and note what voltage drop polarity that represents relative and what it does for the voltage on the negative terminal of the capacitor with respect to ground.
You should see that when if you open the open the rightmost switch and allow the capacitor to charge, then close the switch so the capacitor discharges, the discharge current flows through the 10K resistor in a direction where (using passive sign convention) the resistor terminal for the + end voltage drop is the ground/0V node and the terminal for the negative end of the voltage drop is the capacitor's negative terminal. If you measure with the supply's negative terminal as your reference, then the other end of the resistor (the capacitor's negative terminal) will measure as below ground.