substitute-key-definition problem with menus in Emacs 21

[Drew Adams]

(substitute-key-definition 'customize-group
                           'customize-group-other-window
                           global-map)

If I do this in Emacs 20, then `C-h k' and pick, in menu-bar:
Help>Customize>Specific Group, it tells me that it runs command
customize-group-other-window, as I would expect: "menu-bar help-menu
customize customize-group runs the command customize-group-other-window"

If I do it in Emacs 21, and pick the equivalent Options>Customize
Emacs>Specific Group, it tells me that it still runs command
customize-group: "<menu-bar> <options> <customize> <customize-group> runs
the command customize-group"

Similar uses of substitute-key-definition seem to work in Emacs if the key
binding is not a menu item. It seems broken for menu item bindings in Emacs
21.

What gives? Do I have to use substitute-key-definition somehow differently
in Emacs 21? I can't really tell if this is a bug or just operator
ignorance. I see nothing in the Emacs NEWS that speaks to this.

BTW, I notice that I can now do this as an alternative:

(define-key global-map [remap customize-group]
'customize-group-other-window)

That works. Why doesn't substitute-key-definition still work with menus?

In GNU Emacs 21.3.50.1 (i386-mingw-nt5.1.2600)
 of 2004-07-26 on BERATUNG4
configured using `configure --with-gcc
(3.3) --cflags -I../../jpeg-6b-1/include -I../../libpng-1.2.4-1/include -I..
/../tiff-3.5.7/include -I../../xpm-nox-4.2.0/include -I../../zlib-1.1.4-1/in
clude'