diff --git a/README/ReleaseNotes/v640/index.md b/README/ReleaseNotes/v640/index.md
index 653e1c07ac4c0..bc13f3f01418a 100644
--- a/README/ReleaseNotes/v640/index.md
+++ b/README/ReleaseNotes/v640/index.md
@@ -108,6 +108,9 @@ gInterpreter->Print("autoparsed");
 for example, this can be used to enforce that no header is loaded for I/O operations. To disable the
 auto-parsing during `TClass::GetClass`, you can either set the shell environment variable
 `ROOT_DISABLE_TCLASS_GET_CLASS_AUTOPARSING` (to anything) or set the `rootrc` key `Root.TClass.GetClass.AutoParsing` to `false`.
+* Alignment of classes and numerical types is now recorded in the dictionary and propagated through `TClass`, `TDataType`, and `TStreamerElement`.  `TStreamerInfo::BuildOld` uses this information to correctly lay out emulated classes and older-version StreamerInfos (for the input of I/O customization), ensuring proper alignment for over-aligned types.
+  Fixes [#21667](https://github.com/root-project/root/issues/21667).
+  (PR [#21669](https://github.com/root-project/root/pull/21669))
 
 ## Geometry
 
@@ -184,6 +187,7 @@ See: https://github.com/root-project/root/pull/20963 for implementation details
 
 * The behavior or `TDirectoryFile::mkdir` (which is also `TFile::mkdir`) was changed regarding the creation of directory hierarchies: calling `mkdir("a/b/c", "myTitle")` will now assign `myTitle` to the innermost directory `"c"` (before this change it would assign it to `"a"`).
 * Fixed a bug in `TDirectoryFile::mkdir` where passing `returnExistingDirectory = true` would not work properly in case of directory hierarchies. The option is now correctly propagated to `mkdir`'s inner invocations.
+
 ### File Permissions Now Respect System `umask`
 
 ROOT now respects the system `umask` when creating files, following standard Unix conventions.