The utility network layer is not yet supported in the web map spec. That is why you might have noticed when we publish a map that contains a utility network layer the sublayers are flattened into regular layers (point errors, line errors etc..). When we turn around and add the new web map to Pro the application does not recognize the utility network because it never existed in the web map.
The web map spec is evolving to include the utility network and other new layer types such as the subtype layer. No hard timeline for when those will come.
There are multiple workarounds for this:
1. (For 2.1/2.2) Add the web map to Pro and then go to the original feature service and add the utility network layer (only) to the map. That should activate the ribbon.
2. Use project packages instead of web maps for Pro. Although Pro can consume web maps, the native pro project package will support not only the utility network but any layer that is supported by Pro such as the subtype layer which is much more performant than regular layers. I do understand the desire to use 1 web map that can be served across the entire organization in different clients instead of maintaining multiple maps. However, I think maps should be optimized to take advantage of smart clients capabilities.
In 2.3 we have made a fix to allow displaying the utility network ribbon all the time if at least one layer that belongs to a UN is detected. The workaround to add the utility network layer won't be necessary in this case.
Edit 12-14-2018: The utility network layer still need to be present in the map in 2.3, you just don't have to click on the layer to activate the UN tab. So the workaround to add the utility network layer after opening the web map still required.