The pop-up for the found features shown by the Search widget in Web AppBuilder for ArcGIS does not recognize the hidden ObjectID field configured in ArcMap, when the layer source for the Search widget is added from a service URL.

Solución alternativa

1. In the related web map, add a layer from Web.
2. Hide the added layer in Contents, and save the web map.
3. Go to the Search widget configuration windows.
4. Add the search source but this time select from the map. Click OK.
5. Search for the attribute. There is no more ObjectID.

Pasos para reproducir