Unity cannot serialize nested scriptable object

I want to save a timeline playable asset as a child of a custom ScriptableObject, I do this by following code:

 var data = ScriptableObject.CreateInstance<DataType>();
 AssetDatabase.CreateAsset(data, "Assets/data.asset");
 
 data.timeline = ScriptableObject.CreateInstance<TimelineAsset>();
 AssetDatabase.AddObjectToAsset(data.timeline, data);
 AssetDatabase.SetDirty(data);
 AssetDatabase.SaveAssets();

But when I open the data.asset, it does not serialize any data for the playable asset!

Help!