This speaks to a larger issue that I'm having with publishing feature classes with subtypes for use in web maps on ArcGIS Enterprise as referenced layers. I'm going to keep this simple for the purposes of starting a discussion about best practices when it comes to working with subtypes in a web map.
Scenario #1
I published a layer (with subtypes) from Pro 3.5 to Enterprise 11.5. At first, I published this layer with single symbol symbology. Upon opening in a web map, I noticed that the pop ups were still showing domain values, rather than their labels. Below, you can see the Asset Group is blank (this is the subtype field), and the Asset Type (domain driven by asset group) is a coded domain value.
To fix this, I opted to create a quick attribute expression that decodes the domain.
Asset Group = return SubtypeName($feature)
Asset Type = return DomainName($feature, 'assettype')
Upon testing, this appears to work just fine. However, see the GIF below. The popup previews seems to take, but when the user interacts with the map, the popup forgets the attribute rules.
Scenario #2
I went back to the same layer and published with unique symbology from the main subtype field. This fixed the pop up issue. However, I cannot symbolize on anything beyond single symbology. The issue seems to come and go… for example, if I add the layer to a standalone map, the symbology works just fine. But if I add it to a production map, it fails. Below is a GIF of how the behavior manifests.
I'll admit it's a little hard to see in the GIF. Whenever I click back to the Style pane, the previously configured symbology gets erased.
