If a user adds a service to a map with '[name of service]/mapserver' instead of '[name of service]/MapServer' in the rest URL, Explorer for ArcGIS Online does not load the layer. Explorer for ArcGIS on iOS is able to load the rest URL in lowercase form.
Change the map service URL to end in 'MapServer'.
This can be done by either adjusting the URL in ArcGIS Online Assistant or just re-adding the layer in ArcGIS Online or ArcGIS for Portal.