solved some bugd and pulled from latest preview

Assets/Scenes/ImageTracking/Multi Prefab - Image Tracking/Multi-Image Tracking.unity

   m_Name: 
   m_EditorClassIdentifier: 
   m_PrefabList:
-  - m_Prefab: {fileID: 6916502901855112170, guid: e22b43a8fcdab413bb4f52f5076f759e,
+  - m_ImageGuid: f0340be5-710e-49ff-bc70-45f1315b9202
+    m_Prefab: {fileID: 6916502901855112170, guid: e22b43a8fcdab413bb4f52f5076f759e,
-  - m_Prefab: {fileID: 2804028924010200506, guid: 379785de5e39005469c875710d3cc5a7,
+  - m_ImageGuid: 2c29eeed-d95c-4111-b724-80f9ff10ce86
+    m_Prefab: {fileID: 2804028924010200506, guid: 379785de5e39005469c875710d3cc5a7,
-  - m_Prefab: {fileID: 3279860958548870494, guid: 2584331523bd14e8898965d93034efdb,
+  - m_ImageGuid: aae75228-1973-4dc0-b7a2-53d1566ee402
+    m_Prefab: {fileID: 3279860958548870494, guid: 2584331523bd14e8898965d93034efdb,
       type: 3}
   m_ImageLibrary: {fileID: 11400000, guid: c86e375d7e7430442a1104e88fa21f21, type: 2}
 --- !u!114 &264295717

Assets/Scenes/ImageTracking/Multi Prefab - Image Tracking/MultiTrackedImageInfoManager.cs

         /// </summary>
         public struct NamedPrefab
         {
-            Guid m_ImageGuid;
+            [SerializeField]
+            string m_ImageGuid;
-            public Guid imageGuid
+            public string imageGuid
-                get => m_ImageGuid; 
+                get => m_ImageGuid;
             }
 
             [SerializeField]
             {
-                get => m_Prefab; 
+                get => m_Prefab;
-            public NamedPrefab(Guid imageGuid, GameObject prefab)
+            public NamedPrefab(Guid guid, GameObject prefab)
-                m_ImageGuid = imageGuid;
+                m_ImageGuid = guid.ToString();
                 m_Prefab = prefab;
             }
         }
 
         Dictionary<Guid, GameObject> m_PrefabsDictionary = new Dictionary<Guid, GameObject>();
         ARTrackedImageManager m_TrackedImageManager;
-        
+
         [SerializeField]
         [Tooltip("Reference Image Library")]
         XRReferenceImageLibrary m_ImageLibrary;
         void Awake()
         {
             m_TrackedImageManager = GetComponent<ARTrackedImageManager>();
-            
+
-            {                
+            {
                 m_PrefabsDictionary.Add(ImageLibrary[i].guid, prefabList[i].prefab);
             }
         }
         {
             if (library)
             {
-                if (m_PrefabList == null)
+                if (prefabList == null)
-                    m_PrefabList = new List<NamedPrefab>();
+                    prefabList = new List<NamedPrefab>();
-                        m_PrefabList.Add(new NamedPrefab(library[i].guid, null));
+                        prefabList.Add(new NamedPrefab(library[i].guid, null));
                     }
                 }
                 else
                     {
-                        var idx = m_PrefabList.FindIndex(item => item.imageGuid == library[i].guid);
-                        tempList.Add(new NamedPrefab(library[i].guid, (idx != -1) ? m_PrefabList[idx].prefab : null));
+                        var idx = prefabList.FindIndex(item => item.imageGuid == library[i].guid.ToString());
+                        tempList.Add(new NamedPrefab(library[i].guid, (idx != -1) ? prefabList[idx].prefab : null));
-                    m_PrefabList = tempList;
+                    prefabList = tempList;
-            }  
+            }
         }
 
         void OnTrackedImagesChanged(ARTrackedImagesChangedEventArgs eventArgs)
                 // Give the initial image a reasonable default scale
-                trackedImage.transform.localScale = new Vector3(trackedImage.size.x/2, trackedImage.size.x/2, trackedImage.size.x/2);
+                trackedImage.transform.localScale = new Vector3(trackedImage.size.x / 2, trackedImage.size.x / 2, trackedImage.size.x / 2);
 
                 AssignPrefab(trackedImage);
             }
         {
-            if (m_PrefabsDictionary.TryGetValue(trackedImage.referenceImage.guid, out GameObject prefab)){
+            if (m_PrefabsDictionary.TryGetValue(trackedImage.referenceImage.guid, out GameObject prefab))
+            {
-        #if UNITY_EDITOR
+#if UNITY_EDITOR
 
         [CustomEditor(typeof(MultiTrackedImageInfoManager))]
         public class MultiTrackedImageInfoManagerInspector : Editor 
                 if (EditorGUI.EndChangeCheck())
                     behaviour.OnLibraryChanged(library);
                 else if (HasLibraryChanged(library))
-                    behaviour.OnLibraryChanged(library);
+                    behaviour.OnLibraryChanged(library);     
-                // Update current
+                // update current
                 m_ReferenceImages.Clear();
                 if (library)
                 {
                 }
 
                 serializedObject.ApplyModifiedProperties();
+                Undo.RecordObject(target, "Sync Library");
+                EditorUtility.SetDirty(target);
-        #endif
+#endif
     }
 }

ProjectSettings/ProjectSettings.asset

     16:9: 1
     Others: 1
   bundleVersion: 0.1
-  preloadedAssets:
-  - {fileID: 4800000, guid: e7c77f6eaab324a819efdc13b8125a39, type: 3}
-  - {fileID: -7479341934665344595, guid: e2f869774b12e400996eb546603602e3, type: 2}
+  preloadedAssets: []
   metroInputSource: 0
   wsaTransparentSwapchain: 0
   m_HolographicPauseOnTrackingLoss: 1