Feature services symbolized with some unique fields fail to show the drop-down list for subtypes (coded domain values) when editing the service in a web map.
In 10.5, we added support in the feature service for listing subtype information independently from the type information on a layer. Thus, regardless of the renderer, the service provides the subtype info. Not all clients have consumed it as this point (Pro will likely be the first), but this enhancement addresses the issue on the server side and allows the clients to work as like ArcMap as is described in the issue. For this reason, I am closing this issue.