Per,
This is an interesting question. From how it looks, it looks like this is the intended behavior.
When you create a map in the Map Viewer, it uses the default extent that you selected:
Configure map viewer—Portal for ArcGIS (10.4) | ArcGIS for Server
For Default Basemap, choose the basemap you want opened when users create a new map with the map viewer. You see the maps in the group you selected for the Basemap Gallery. The default basemap opens at the default extent you set. To set the default extent to the extent of your currently selected default basemap, click Use map extent as default. To set a different extent, click Choose Extent and enter numerical values or draw the extent. A geocoder is included to help you find a specific place or address. |
When you're just adding an item, it will initially go to the extent of that item:
Item details—Portal for ArcGIS | ArcGIS for Server
Extent—The extent is the geographic boundary you specify for the item. It is defined using the minimum and maximum x and y coordinates of the area. You can either type these coordinates or draw a box to define the area. The draw option includes a geocoder to help you find a place or address on the map. Portal for ArcGIS updates the coordinates of the extent based on what you draw. |
Though, it looks like you may be able to set up some advanced options within Portal and use the "extent service" to configure your extent:
Set advanced portal options—Portal for ArcGIS (10.4) | ArcGIS for Server
Do you have a WKID that you can use for this?
