The most common cause of this that I've seen is when your web map is referencing a feature service that includes subtype group layer or additional renderers that affect the web map's ability to interpret coded values.
Add your utility network to a blank map, clicking yes when asked to add all the layers it contains, and use that to publish your feature service. Point your web map to this feature service (if you matched the layer ids of your previous service that will make this easier) and the issue should be resolved.
If you look at the maps provided by the Utility Network Foundation(s) you will see that this is how they built their maps (X Utility Network = Feature Service, X Editor = Desktop Map, Viewer = Web Map) with all the symbology and label expressions defined in the desktop and web maps and the feature service being very simple.
