if the logic is the same as the 45e, the following post has links to multiple threads on the subject.
https://www.bimmerpost.com/forums/sh...php?p=29575586
in a nutshell, the vehicle will turn off when the driver exits the vehicle. this is to ‘prevent engine idling’ which is a logic carried over from ICE-only vehicles even though the 45e/50e are PHEV.
it takes some planning and behavior modification, but if the goal is to keep passengers comfortable while the driver steps out quickly, ensure you have adequate HV battery charge so you can activate preconditioning before stepping out (should pop up on the control display after pressing to Stop button to end your trip and
must be done before the driver opens the door).
there are a couple workarounds to actually idle the engine, though. steps are buried in the link above.