Known Limit
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
Additional Information
This is currently a limitation of the 4.x series of the ArcGIS API for JavaScript. The application supports labeling once the API supports labeling.
For more information, refer to https://developers.arcgis.com/javascript/latest/api-reference/esri-layers-FeatureLayer.html#labelsVisible