The exported CSV file of the related data only includes the ObjectID field from the Map widget in ArcGIS Experience Builder.

Solución alternativa

You can export the data using the "View in table" option. Please follow these steps: 

1. Click on the feature.
2. Click on Actions.
3. Select Related Data.
4. Click on the table.
5. Go to Actions and select the View in Table option.

1. Expand the table beneath the map.
2. Navigate to Actions > Export.
3. Select the option Export to CSV.

Pasos para reproducir