Not sure how relevant this is, but curious readers might be interested: In those 400-800 volt systems, the low-voltage system is used to wake up the battery's computer and close a normally-open contactor inside the battery that connects the high-voltage system.
So indeed they might not ground HV stuff to the chassis, and the hundreds-of-volts sources are disconnected if the car is turned off and unplugged.
Exactly. Apart from waking up the vehicle and closing the high voltage connectors, a whole myriad of things run off the low-voltage system, including lights, climate, infotainment and most (if not all) actuators.
So indeed they might not ground HV stuff to the chassis, and the hundreds-of-volts sources are disconnected if the car is turned off and unplugged.