What concerns me, of course I may be mis-understanding this thread, but I was under the impression that if connection was lost, the Mavic automatically did a RTH, so if the APP crashed to me the best thing to do if the mavic is out of sight is to hit RTH before restarting the APP, at least until you can see it again then restart the APP and regain control if needed.
If you believe a RTH is needed, indeed hold the button down for 3 seconds before shutting down/re-starting the App.
However, the RTH Home Point is stored On the Drone . So an app re-start does not reset the Home Point. That required deliberate user action or a re-start of the drone itself.
The automatic setting of the Home Point belongs to the drone when it receives GPS position after startup. That position is relayed to the App (via the RC) to tell the pilot it has been done and for display on the map,
Which is why it is recommended to wait for GPS position before starting the motors.