help for git format-patch lost diffstat

Hi, 

Here are steps of what I am doing:

First, I created two commits to a demo project.

when I use command: git format-patch --cover-letter -1

cat 0000-cover-letter.patch

From 2e62ef46d5627319c4cb11be81079e3364fca40c Mon Sep 17 00:00:00 2001
From: FanJun Kong <bh1scw@gmail.com>
Date: Sun, 24 Apr 2022 14:31:45 +0800
Subject: [PATCH 0/1] *** SUBJECT HERE ***

*** BLURB HERE ***

FanJun Kong (1):
  Second commit.

   1.c | 4 ++++
   1 file changed, 4 insertions(+)

-- 2.32.0 (Apple Git-132)

ok, you can see the cover letter has diffstat, just above "--".

Then I change command to:

git format-patch --cover-letter -2

cat 0000-cover-letter.patch

From 2e62ef46d5627319c4cb11be81079e3364fca40c Mon Sep 17 00:00:00 2001
From: FanJun Kong <bh1scw@gmail.com>
Date: Sun, 24 Apr 2022 14:34:48 +0800
Subject: [PATCH 0/2] *** SUBJECT HERE ***

*** BLURB HERE ***

FanJun Kong (2):
  First commit.
  Second commit.

-- 2.32.0 (Apple Git-132)

Comparing with last command, I just want to get 2 patches, but this time the diffstat is missing.

I am not sure if this is a bug or I miss some options.

I googled this issue, but not got too much.

Thanks for your time. Fanjun Kong