Fix BoxSet collapse handling and deletion

MediaBrowser.Controller/Entities/CollectionFolder.cs

@@ -79,14 +79,27 @@ namespace MediaBrowser.Controller.Entities
         public CollectionType? CollectionType { get; set; }
 
         /// <summary>
-        /// Gets the item's children.
+        /// Gets or sets the item's children.
         /// </summary>
         /// <remarks>
         /// Our children are actually just references to the ones in the physical root...
+        /// Setting to null propagates invalidation to physical folders since the getter
+        /// always delegates to <see cref="GetActualChildren"/> and never reads the backing field.
         /// </remarks>
         /// <value>The actual children.</value>
         [JsonIgnore]
-        public override IEnumerable<BaseItem> Children => GetActualChildren();
+        public override IEnumerable<BaseItem> Children
+        {
+            get => GetActualChildren();
+            set
+            {
+                // The getter delegates to physical folders, so invalidate their caches.
+                foreach (var folder in GetPhysicalFolders(true))
+                {
+                    folder.Children = null;
+                }
+            }
+        }
 
         [JsonIgnore]
         public override bool SupportsPeople => false;