Its questionable if attached properties can be used in markup extensions. In either case, prefix should not be used for a normal property in markup extensions.

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.XamlDom/Project/XamlProperty.cs

@@ -363,11 +363,8 @@ namespace ICSharpCode.WpfDesign.XamlDom
 
 		internal string GetNameForMarkupExtension()
 		{
-			string name;
+			if (IsAttached) {
-			if (IsAttached)
+				string name = PropertyTargetType.Name + "." + PropertyName;
-				name = PropertyTargetType.Name + "." + PropertyName;
-			else
-				name = PropertyName;
 				
 				var element = ParentObject.XmlElement;
 				string ns = ParentObject.OwnerDocument.GetNamespaceFor(PropertyTargetType);
@@ -377,6 +374,9 @@ namespace ICSharpCode.WpfDesign.XamlDom
 				else
 					return prefix + ":" + name;
 			}
+			else
+				return PropertyName;
+		}
 		
 		/// <summary>
 		/// used internally by the XamlParser.