Capacitor problem using an NMOS inverter with depletion load
The depletion mode device is ON when its Vgs = 0, as in your case. So it acts like a small resistor through which the capacitor can charge, even if M1 is off.

A depletion mode N-mosfet has a channel already formed in the silicon and doesn't need additional gate drive to turn it on. In fact you must apply a negative Vgs to turn it off (its threshold voltage is negative)!