diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManager.php b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
index e48b078..243d02a 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManager.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
@@ -253,6 +253,9 @@ public function getPropertyInstance(TypedDataInterface $object, $property_name,
     if ($definition instanceof ListDataDefinition) {
       $definition = $definition->getItemDefinition();
     }
+    // We consider we can reuse an existing prototype if it was built:
+    // - for the same runtime $definition object,
+    // - and for the same parent hiararchy.
     $key = spl_object_hash($definition) . '.';
     $key .= ':' . $object->getPropertyPath() . '.';
     // If we are creating list items, we always use 0 in the key as all list