So the HV battery doesn't try to keep the 12v one topped up while standing. Just trying to get my head around it.
Shouldn't be a problem as I frequently do 2 x 30 mile journeys weekly. As opposed to daily before I retired.
I'm sure the big battery is isolated when the car is switched off to avoid any risk of the battery draining completely by topping up the 12V battery. If a lithium battery is left fully discharged for any significant period then it's bad for the chemistry and very expensive to replace this battery. The same also applies to the lead-acid battery but it's much cheaper to replace.
The 12V battery doesn't need to be much more than a big capacitor to serve the various components which want 12V power. There's no longer the starter motor to supply (that work is done by the big motor-generator fixed to the engine), most of the lights are LED (so no longer a pair of 60W headlight bulbs) and there are the various bits of electronics which must also benefit from the relatively stable voltage of the 12V battery compared to the fluctuations that the big battery must experience as the drive system changes modes.
However, as the OP has reported, there seems to be a situation where the 12V battery can be drained when the vehicle has been standing for a few days when it should have the capacity to supply for many weeks the residual power demands from components still alive when the vehicle is off (locking system, telemetry, ...). So far, this is a one-off event for which it would be useful to know the cause. I can't be the only person to have had one of these vehicles parked up for many weeks during the lockdowns either side of Christmas. My topping up of the 12V battery was more through habit than necessity.