I have deployed a Water, Sewer, and now Electric Utility Networks. On my Water and Sewer I took the default Network Editor maps and pointed it to my SDE and then published a Map Image Layer for View only for Web Applications (no editing capabilities). I didn't change any of the subtypes within the map, I just added labels and scales. However, when I do the same thing for Electrical the Web Map in Portal is showing my coded domain values instead of the name, see below of what I except it to look like and what I get. Is there a setting somewhere within the Pro project that will allow this when publishing that it would have been set for Water and Sewer, but not Electrical? I checked the share layer window and the settings all seem to be the same. I'm using Pro 2.6.7 and Enterprise/Portal 10.8.1 for this.
Also, the feature layer and map image layer that was shared for editing capabilities with the Electric Utility Network map pulls up the labels correctly in a Portal Web Map.