bug#72080: imenu returns different lists in emacs-news-mode and emacs-news-view-mode

[Siyuan Chen <chansey97@gmail.com>]

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Reproduce steps:

1 Clone Emacs source code from https://github.com/emacs-mirror/emacs

2. Emacs -Q (Emacs 29.3 on Windows)

3. Open etc/NEWS.30 from the Emacs source code (by default Emacs will use
emacs-news-mode)

4. M-x imenu and press TAB to complete

Emacs shows 515 possible completions.

6. M-x emacs-news-view-mode

7. M-x imenu and press TAB to complete

Emacs shows 202 possible completions.

This is the problem, i.e. emacs-news-mode and emacs-news-view-mode return
different imenu lists.

The Background:

I am reading Emacs NEWS and using the "imenu-list" package to display the
outline of the NEWS as a side window since NEWS are often very long. It
works well in emacs-news-mode, but lacks many entries in
emacs-news-view-mode. BTW the emacs-news-view-mode is nice because it
allows button click symbol references in the buffer..

In step 4 and step 7, you can do M-x eval-expression

```
(progn
  (require 'imenu)
  (setq message-log-max t)
  (let ((inhibit-read-only t))
    (with-current-buffer "*Messages*" (erase-buffer)))
  (pp (imenu--make-index-alist)))
```
and save the *Messages* buffer to two files respectively, e.g. news-mode.txt
and news-view-mode.txt. You will find that these two files are different.
The news-view-mode.txt lacks many imenu entries.

P.s. This issue only occurs for NEWS.30. It is OK for NEWS.29.

Thanks.

Best regards,
Siyuan Chen

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