Why Doesn't SPICE Show the Discharging of a Capacitor after Input Voltage is Removed
You might think that setting your PWL to zero makes it open circuit. It doesn't. It's generating a low impedance zero, that discharges the capacitor.

If you want to simulate removing it, then you need a switch in series with it that opens at some time, or a variable resistor that you set to a very large value.

Series diodes are often used in real life so that a collapsed main power supply doesn't drag down its backup supply. You could do the same here.