The print area does not correspond with the extent of the ArcGIS Online Map Viewer export after adjusting the DPI when the ‘Set scale’ option is enabled.
Set the extent of the print area to the desired extent while the DPI is still at 96.
Change the DPI to the desired value, disregarding the extent box enlargement, as the correct extent is maintained when exporting the map.
Export the map and revert the DPI to 96 to choose a new area. This ensures the print preview is correctly aligned for the first and subsequent exports.