ScriptableObject value not persisting after OnImportAsset

I have a scripted importer that invokes the following code upon an imported asset:

Code (CSharp):

1. var novelObject = ScriptableObject.CreateInstance<NovelObject>();
2. novelObject.RootNode = walker.Walk();
3. novelObject.Test = "value";
4.  
5. ctx.LogImportWarning($"Test value: {novelObject.Test}");
6. ctx.LogImportWarning($"RootNode is null: {novelObject.RootNode == null}");
7.  
8. ctx.AddObjectToAsset("novel object", novelObject);
9. ctx.SetMainObject(novelObject);

With this ScriptableObject being instantiated:

Code (CSharp):

1. [Serializable]
2. public class NovelObject : ScriptableObject
3. {
4.     public INovelObject RootNode;
5.     public string Test;
6. }

And later, in a MonoBehaviour attached to a GameObject in my scene, I have:

Code (CSharp):

1. public class NovelFrameController : MonoBehaviour
2. {
3.     [SerializeField] private NovelObject _novelObject;
4.  
5.     private void Start()
6.     {
7.         Debug.Log($"Test value: {_novelObject.Test}");
8.         Debug.Log($"RootNode is null: {_novelObject.RootNode == null}");
9.     }
10. }

Note my calls to ctx.LogImportWarning and Debug.Log.

After letting this compile, I reimport a test asset and run my game, with the following output:



At import time, Test and RootNode both have a non-null value. However, at Runtime, the value of RootNode is set to null. Why?

 

if INovelObject is an interface, Unity can't serialize interfaces by default. It is then cleaned up and null during runtime.