To enable backgrounding on iOS, set an additional property called AllowsBackgroundLocationUpdates to true: https://developers.arcgis.com/net/api-reference/api/ios/Esri.ArcGISRuntime/Esri.ArcGISRuntime.Location.SystemLocationDataSource.AllowsBackgroundLocationUpdates.html
Additionally, check out the new Backgrounding and Location Tracking sample application, which demonstrates backgrounding and location workflows: https://github.com/Esri/arcgis-maps-sdk-dotnet-demos/tree/main/src/BackgroundLocationTracking