On Fri, May 6, 2022 at 1:02 PM Junio C Hamano [off-list ref] wrote:

And this is supposed to add a new entry to the index at
"git-test-123456/test-file"?

Correct, I expected an index entry at "git-test-123456/test-file".  If
I run the following:

test_repo="$(mktemp -d /tmp/git-test-XXXXXXX)"
cd "$test_repo"
>test-file
>/tmp/test-file
git init
git config core.worktree /tmp
git add test-file
git add /tmp/test-file

.. then with current git I get an index entry for "/tmp/test-file" but
not "/tmp/git-test-123456/test-file".  That is, I can add files
outside the standard worktree location but not inside it.

A playground example of the prior behavior (git 2.25.1) is here:
https://www.onlinegdb.com/OXt2cvL8S