Inconsistent Behavior in 'git add' (git 2.52.0)

A while back (6 Sep 2025) I submitted a report about how running

	git add x bogus_file

where 'x' exists but 'bogus_file' doesn't results
in 'x' not being added to the index. Peff was kind
enough to explain that the presence of even 1 invalid file invalidates
the whole 'git add' command, no matter how many valid files
are included. OK, fine.

This morning I was experimenting with what happens if a file
is specified on a 'git add' command line where the file exists
but is in a .gitignore file. I was expecting the same behavior
as when the file doesn't exist at all. Here's what happened:

% ls -l file1 jon
-rw-r--r-- 1 jonf 15 Dec 30 15:36 file1
-rw-r--r-- 1 jonf  0 Jan  4 11:19 jon

% git check-ignore -v file1
.gitignore:2:file1      file1

So far, so good. Both file1 and jon exist, but file1 is
in .gitignore. Just for yuks, I start with a clean repo.

% git init
Initialized empty Git repository in /tmp/fish/.git/

The index is empty, as shown by
% git ls-files --cached
%

I then ran

% git add file1 jon
git add file1 jon
The following paths are ignored by one of your .gitignore files:
file1

Again, this is exactly what I expected. But, running
'% git ls-files --cached' shows

jon

This is *not* what I expected. I expected the 'git add' command to
not add anything to the index, the same way it behaved when I tried
to add a file that doesn't exist (see the beginning of this message).
But, apparently specifying a missing file is considered a different
kind of error than specifying an ignored file.

Once again, I'm wondering if this is expected behavior.

Cordially,
Jon Forrest