Think of it as a sealed plate in the cylinder: the pressure sensor is below the plate.
Pre-charge 1000psi, sensor reads 1000 psi
Normal Ops 3000 psi ABOVE the plate compressess the pre-charge gas to 3000psi and sensor reads 3000psi
Lose the pre-charge - the plate moves to the bottom of the accumulator as soon as pressure is applied above.. No matter what pressure is put in above, sensor reads 0 - no gas and no space to pressurise the gas.