Steps to reproduce:

1. Create a custom Graph type that supports subgraphs

{quote}[Graph(extension:"abc", GraphOptions.SupportsSubgraphs)]
public class GraphTest : Graph
{
    [MenuItem("Assets/Create/Graph/GraphTest")]
    public static void CreateGraph()
   
Unknown macro: \{         GraphDatabase.PromptInProjectBrowserToCreateNewAsset<GraphTest>();     }
}
{quote}

1. Create two assets of your Graph type using the project window
2. Double click one of the graphs to open a graph window
3. Drag and drop the second graph asset into the graph window to create a subgraph node
4. Save the graph and close the window
5. Re-import a script in your project to trigger a domain reload
6. Observe the console

Reproducible with versions: 6000.6.0a1

Not reproducible with 6000.5.0a8, 6000.4.0b11

Tested on (OS): MacOS