Verbose commit message diff not showing changes from pre-commit hook

Hi,

I'm using git version 2.27.0 on Linux.

I have verbose commits enabled (`git commit --verbose`) and a
repository configured to lint code before every commit with a
pre-commit hook. This hook may change files and `git add` them just
before the commit.

However, when that hook actually changes files (and `git add`s them
right away) these changes are not reflected in the commit verbose diff
(the commented lines below the commit message). The changes *are*
taken into account by git, as `git diff --staged` shows — and later
the commit info, after actually making the commit. However the
displayed diff in the commit message file is a snapshot of the diff
*before* the pre-commit hook.

Is this expected behaviour? I find it somehow confusing that the diff
in the commit message isn't the actual commit diff.

Thank you for your help!

Kind regards,

-- 
Maxime Louet