From a Support Services case that seems similar to yours:
Resolution: To edit a Utility Network you need to register your data as branch versioned and publish it to your organization's portal.
As stated in Esri's documentation:
From http://pro.arcgis.com/en/pro-app/help/editing/edit-a-utility-network.htm
"A utility network can only be edited in a map with a published feature service registered as branch versioned"
From https://pro.arcgis.com/en/pro-app/help/data/geodatabases/overview/data-management-strategies.htm#ESR...
"Branch versioning is the mechanism behind long transactions for services. If you require multiple editors concurrently accessing services with the ability to undo and redo their edits, you must first register your data as branch versioned'
Using branch versioning "You can edit features in a utility network. This service-based dataset is only available using a Web GIS deployment"
Branch versioning "Can be used only in ArcGIS Pro and edited only with feature services."
"Simple features and the utility network are currently the only datasets available for editing through branch versioning."
Here's a couple of links you might find useful for future reference:
Register a dataset as branch versioned
https://pro.arcgis.com/en/pro-app/help/data/geodatabases/overview/a-quick-tour-of-registering-and-un...
Share branch versioned data
https://pro.arcgis.com/en/pro-app/help/data/geodatabases/overview/publish-branch-versioned-data.htm
Branch Versioning in the Utility Network
https://sspinnovations.com/blog/versioning-dummies-pt-5-branch-versioning-utility-network/