The Comment button disappears from the Crowdsource Reporter web application if feature services are configured with the 'Limit Access to Public ArcGIS Online Layers' functionality.
The service in this case had been published without the relationship that is required for comments to work. Once the service was published with the relationship intact, the issue was resolved.
対処法
If layers are not configured with the 'Limit Access to Public ArcGIS Online Layers' function, the Comment button displays and works as expected.