UnityEvent Serialization Problem

Hello, I have an issue relating to using UnityEvents in a Custom Inspector.

I've managed to get it into some shorter code.

This is my script:

 using UnityEngine;
 using UnityEngine.Events;
 
 public class Test : MonoBehaviour {
 
     [SerializeField]
     public MyEvent myEvent;
 
     [System.Serializable]
     public class MyEvent : UnityEvent { }
 }

On its own the script works as expected. But when using the Custom Inspector script bellow there is a problem.

 using UnityEngine;
 using UnityEngine.Events;
 using UnityEditor;
 using UnityEditor.SceneManagement;
 
 [CustomEditor(typeof(Test))]
 public class TestEditor : Editor {
 
     public override void OnInspectorGUI() {
         GUILayout.Label("This is a label just to show the Custom Inspector is working.");
         Test myTarget = (Test)target;
         SerializedObject s = new UnityEditor.SerializedObject(myTarget);
         s.Update();
         EditorGUI.BeginChangeCheck();
         EditorGUILayout.PropertyField(s.FindProperty("myEvent"), true);
         s.ApplyModifiedProperties();
         if (EditorGUI.EndChangeCheck())
         {
             EditorUtility.SetDirty(target);
             EditorSceneManager.MarkAllScenesDirty();
         }
     }
 }

The problem is when entering a value for a static parameter into the Editor. For example, I can add a method to call such as 'GameObject > int layer' and enter a value like bellow.

Then I can click on another GameObject in the scene. When I go back to this script the value gets replaced with 0 (as bellow). I assume it is not being serialized somehow. But the default Unity Editor can serialize it so it must be possible. Is my code wrong?

Thank you