There is no 1:1 connection between blocks/zones/page on frontend and backend due to how page fieldtype storage is implemented. Currently frontend is assigning UUIDs to block/zones and after it's saved, all ids are changed to numerical on the database layer.

This leads to many issues i.e. you can't refresh page preview iframe. In order to fix the issue we need to streamline block identification method. My recommendation is to use UUIDs assigned by frontend (think how to manage collision during save) and save these in the database.

Changing the behavior now will break compatibility thus this story is dedicated for 4.0.