Tableau is adding a new Desktop/ Web Authoring capability that automatically creates and updates Phone layouts for new dashboards when they are edited. This capability does two things 1) automatically adds a new Device Specific Dashboard (Phone layout) when you create a new dashboard, and 2) allows the phone layout to be automatically updated as the Default layout is modified.
This capability is turned on by default for new dashboards within an existing workbook or a new workbook. We don't create phone layouts "automatically" for existing dashboards but an author can add phone layout using Device Preview. The Author retains control on a per-workbook basis whether to disable this capability.
Please let us know if you have any feedback around workbook level control.