generated unique slug should be part of diff

src/PersistentData/PersistentDataManager.php

@@ -115,7 +115,7 @@ class PersistentDataManager implements IPersistentDataManager
 
             if (count($modified) > 0) {
                 if ($model instanceof ModelWithSlug && isset($modified['slug'])) {
-                    $modified['slug'] = $this->generateUniqueSlug($model, $modified['slug']);
+                    $diff['slug']['new'] = $modified['slug'] = $this->generateUniqueSlug($model, $modified['slug']);
                 }
 
                 $modify->setId($id);