David,
At the current time, unfortunately, there is a known limitation with using the Display Flow Direction Arrows UI command with Trace Network Services.
Flow direction is persisted as a network attribute with TN version 2 and later so this can still be used to display flow direction by changing the symbology of the lines in the network to use this field to symbolize the lines. To make this easier I've attached a sample .lyr file containing the default symbology for flow direction as you would see within a TN on a file or mobile geodatabase. To apply this:
1. Right-click the line feature class in the trace network for which you want to display flow arrows and select Symbology.
2. In the Symbology pane that opens, select the Flow direction field under Unique values to display the 3 symbol classes and then select Import Symbology from the burger button at the top right.
3. In the Apply Symbology From Layer GP tool, set the class you want to apply symbology to in the Input Layer (should already be set) and navigate to the FlowDirectionSymbology.lyrx provided as the Symbology Layer.
4. Set the Symbology Fields as below and leave others as default, nd click Run:
- Type: Value field
- Source field: FlowType
- Target Field: Flow direction
This updates the symbology of the lines class to use the flow arrows imported. I use this layer (Alias renamed to Arrows in below screenshot) to display flow by toggling the layer on and off as needed in the Contents pane. I also disable the selectability on this layer. I then use another copy of the line for general display.
We are working to address this in a future release but I hope this helps you move forward with the ability to display flow in the meantime.
Jon