[O] [beamer] blank outline slide?

[zwz]

In a org file, I have

* item1
** aaa
*** bbb
    123
* item2
** ccc
*** ddd
    123

The generated slides have just a blank outline page.
However, if I modify the org content as

* item1
** aaa
*** bbb
* item2
** ccc
*** ddd

The outline page contains item1 and item2, just as expected.

here is my settings:

#+startup: beamer
#+LaTeX_CLASS: beamer
#+LaTeX_CLASS_OPTIONS: [presentation]
#+BEAMER_HEADER_EXTRA: \usetheme{Madrid}\usecolortheme{default}
#+BEAMER_FRAME_LEVEL: 2
#+COLUMNS: %40ITEM %10BEAMER_env(Env) %9BEAMER_envargs(Env Args) 
%4BEAMER_col(Col) %10BEAMER_extra(Extra)
#+OPTIONS:   H:3 num:t toc:t \n:nil @:t ::t |:t ^:nil -:t f:t *:t <:t

Is there anything wrong with my settings or is it a bug?