Ie is the pressure sensor in the line or the bottle?
Oxygen cylinders never have pressure sensors in them. All monitoring is downstream of the isolation valve that allows you to change the cylinder. There must be a bit of line between the cylinder and the on/off valve, so it is totally possible for that bit of line to be pressurised and then the cylinder turned off.
DOI PPL, not big jets, but working every day with oxygen (and other gases) supply systems.