Unity custom editor, values will not serialize

I'm aware this problem has been asked like 1000 times. As far as I can tell, though, my setup is fine, and I've done this before. I have the usual issue: I edit my object, but it will not keep those values when I reload the scene. Interestingly enough, it appears to sometimes keep the values when I go to play mode, which as I understood before should have meant it was working. Guess not.

Another super weird thing: when I am editing an object that's in the scene view (selected from the hierarchy window), the editor shows up properly in the inspector and I can do whatever. But when I try to edit the prefab (selected from the project window), the inspector shows absolutely no properties, just the script name that I can tab open or closed (to no effect).

So here's my (partial) code:

 public class CritterView : MonoBehaviour
 {
     [SerializeField]
     protected List<CritterVisualPart> visualParts;
     
     protected Animator animator;
     
     public void Start()
     {
         //WARNING - before this happens, you should have set the Random.seed to be what you wanted!
         
         if ( visualParts == null )
         {
             visualParts = new List<CritterVisualPart>();
         }
         
         PopulatePartsRandomly();
     }
     
     public List<CritterVisualPart> VisualParts
     {
         get
         {
             return visualParts;
         }
         set
         {
             visualParts = value;
         }
     }

 [System.Serializable]
 public class CritterVisualPart
 {
     [SerializeField]
     protected string name;
     
     [SerializeField]
     protected string copiedPartName;
     
     [SerializeField]
     protected List<CritterVisualOption> options;

 [System.Serializable]
 public class CritterVisualOption
 {
     public enum Type
     {
         Sprite = 0,
         Prefab,
         Count
     };
     
     [SerializeField]
     protected GameObject prefab;
     
     [SerializeField]
     protected Sprite sprite;
     
     [SerializeField]
     protected Type type;

And the editor:

 [CustomEditor(typeof(CritterView))]
 public class CritterViewEditor : Editor
 {
     public override void OnInspectorGUI()
     {
         CritterView critterView = target as CritterView;
         
         List<CritterVisualPart> parts = GetNewParts( critterView );
         
         for ( int partIndex = 0; partIndex < parts.Count; partIndex++ )
         {
             ShowVisualPartGUI( parts, parts[ partIndex ] );
             
             GUILayout.Space( 16 );
         }
         
         critterView.VisualParts = parts;
         
         EditorUtility.SetDirty( critterView );
     }

Is it because I'm nesting serialized objects or something? As far as I know that should work.

Also, does anyone know where I can actually go look at the YAML that gets generated for the prefabs? Doesn't appear to be in the meta file. Seems like that would be a good way to diagnose issues.