Thank you for reaching out. Is the feature service that is being used in the application coming from ArcGIS Server/Portal for ArcGIS or ArcGIS Online?
Thanks for providing the application. I did some testing with the application and was able to see the issue that you are seeing as well. I found that the attachments would not be recognized in the attachment viewer application when there was a map server service in the map, even if this isn't the layer with attachments. I went into my map and adjusted the URL for the map server to reference a direct item in the service, for example: https://<Server_URL>/arcgis/rest/services/<Service_Name>/MapServer/0
Rather than using the map server URL like this: https://<Server_URL>/arcgis/rest/services/<Service_Name>/MapServer
Can you try that adjustment and let me know if that helps the application to recognize the attachments in the water curb stop valves layer?