Indeed.

  cd $(mktemp -d)
  git init
  echo bar > file.txt
  git add file.txt
  git commit -m "init"
  echo foo_bar_baz > file.txt
  # select only "foo_" in magit-status and press "s"
  git diff --cached

will show

  diff --git a/file.txt b/file.txt
  index 5716ca5..2fd000c 100644
  --- a/file.txt
  +++ b/file.txt
  @@ -1 +1,2 @@
   bar
  +foo_bar_baz

rather than the desired

  diff --git a/file.txt b/file.txt
  index 5716ca5..2fd000c 100644
  --- a/file.txt
  +++ b/file.txt
  @@ -1 +1,2 @@
  -bar
  +foo_bar

I can't really imagine this feature working that well with just selection. Editing of the diff seems required.

I just updated magit, too. It seems to have been published 3 days ago.