When the current extent is close to INTL dateline, changing to/from a custom basemap causes the Portal for ArcGIS Map Viewer to zoom out to the world extent.
The custom basemap service extent must include the area that you are zoomed to on the map or, when you switch to the basemap, the map zooms out to the service extent.
If users are experiencing a case where users are zoomed to an area, and switch to a custom basemap, the map zooms to a different extent, update the basemap service extent under the web map’s item details to include the area users are working in.