diff --git a/src/restrictededitingmodeediting.js b/src/restrictededitingmodeediting.js
index 93578c2d..67e451fd 100644
--- a/src/restrictededitingmodeediting.js
+++ b/src/restrictededitingmodeediting.js
@@ -383,6 +383,8 @@ function isRangeInsideSingleMarker( editor, range ) {
 //
 // Note: This marker fixer only consider case which is possible to create using StandardEditing mode plugin.
 // Markers created by developer in the data might break in many other ways.
+//
+// See #6003.
 function ensureNewMarkerIsFlat( editor ) {
 	const model = editor.model;