`git stash --keep-index` includes changes that are on the index

problem demonstration

###
# It is suggested to use the `--keep-index` option when using `git stash` if you don't want to stash
# changes already added to the index.
# This almost works as it leaves the index in the working tree untouched, but it includes the index tree in the patch that's stashed.
###
(>'o')>$ git init
^('o')^$ touch a
<('o'<)$ git add a
^('o')^$ git commit -am "initial commit"
[main (root-commit) 372d94c] initial commit
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a
###
# Initial commit created with an empty file `a`
###
(>'o')>$ echo "Foo
> 
> bar
> 
> baz" > a 
^('o')^$ git add a
###
# File is populated and changes added to the index
###
<('.'<)$ sed 's/Foo//' -i a
^('.')^$ git diff
diff --git a/a b/a
index b5d6cbf..e6b3d90 100644
--- a/a
+++ b/a
@@ -1,4 +1,4 @@
-Foo
+
 
 bar
###
# File is changed more and diff shows how the file differs from the index (line `Foo` replaced with blank line)
###
(>'.')>$ git stash --keep-index
Saved working directory and index state WIP on main: 372d94c initial commit
^('.')^$ git stash list --patch
stash@{0}: WIP on main: 372d94c initial commit

diff --git a/a b/a
index e69de29..e6b3d90 100644
--- a/a
+++ b/a
@@ -0,0 +1,5 @@
+
+
+bar
+
+baz
###
# Unexpectedly, the stash contains the entire file a (all changes on the index as well as the changes in the working directory
# Desired behavior is that the stashed patch should match the `git diff` up above.
###
git commit -a "added bar and baz"
fatal: paths 'added bar and baz ...' with -a does not make sense
(>'.')>$ git commit -am "added bar and baz"
[main dc3b472] added bar and baz
 1 file changed, 5 insertions(+)
^('.')^$ git stash pop
Auto-merging a
CONFLICT (content): Merge conflict in a
The stash entry is kept in case you need it again.
<('.'<)$ git diff
diff --cc a
index b5d6cbf,e6b3d90..0000000
--- a/a
+++ b/a
@@@ -1,4 -1,4 +1,8 @@@
++<<<<<<< Updated upstream
 +Foo
++=======
+ 
++>>>>>>> Stashed changes
  
  bar
^('.')^$
###
# Because the stash includes more than the diff, it pops uncleanly
###