In Portal for ArcGIS Map Viewer, switching to a custom basemap causes the map to zoom to the full extent of the basemap. In some cases, the data in the map is no longer visible as the map extent is changed to the basemap's full extent.
This issue occurs because the custom basemap extent is set to a specific area and does not cover the globe.
Set the extent of the custom basemap to cover the globe. To achieve this, change the extent of the custom basemap web map according to the following coordinates: