Object says it's null, but on inspecting it seems to have the correct value

Hi, I have a derived class that I'm serializing. If I check the value in code, it says it's null. But if I inspect the value it actually has a value. See attached image. Can anyone tell me what's causing this?

As you can see, the if-check succeeds and the for-loop is short-circuited with a 'continue', however 'item' actually has a value, the value that I'd expect it to have.

 [Serializable]
 public class ScriptableActionsAsset : NarrationScriptableObject
 {
     [SerializeField]
     public ScriptableActionItem[] Items;
 }

 [Serializable]
 public class PlayAudioActionItem : ScriptableActionItem
 {
     [SerializeField]
     public AudioClip pathToAudioClip;
 }

 [Serializable]
 public class ScriptableActionItem : ScriptableObject
 {
 }

I realise that the [Serializable] attributes are a bit redundant, but I had them in initially and haven't removed them yet. The ScriptableActionItem and NarrationScriptableObject contains methods but no data, they both extend ScriptableObject.

The ScriptableActionsAsset is a property on a MonoBehaviour. The Start() method of MonoBehaviour (attached to a GO) runs the following method:

 public void ConvertActionItems(GameObject target, IList items)
 {
     foreach(ScriptableActionItem item in items)
     {
         if (item == null)
         {
             continue;
         }
         
         var scriptableItem = item.ToAction(target);
         Actions.Add(scriptableItem);
     }
 }