Secondary menu empty (2nd level in hierarchy)

I tried to use the special feature described in the settings as:
"An advanced option allows you to use the same source for both Main links (currently Inhaltsmenü) and Secondary links: if your source menu has two levels of hierarchy, the top level menu links will appear in the Main links, and the children of the active link will appear in the Secondary links."

In this case the secondary menu is empty (and, as a consequence, does not show up);
when I set the source of the secondary menu to a different menu then it works as expected.

I saw this issue in beta1 as well as in the current CVS version, whereas it worked for me in alpha7.

Steps to reproduce:

- create a new menu.
- add at least three links (grandparent - parent -child).
- configure menu settings: "Source for the Main links" and "Source for the Secondary links" to the created menu.

-> the main links show up (grandparent item), but the secondary one (with the parent item) doesn't.
If I set the secondary links source to a different menu then both show up with the expected content
(both show the top level of their condigured menu).

Looking at the current version I noticed that the value returned by
menu_tree_page_data with max_depth == 2 does not contain the items of the second level of the menu.

The same problem exists with the menu_block module so I think there is a common root cause in menu.inc.

NB: As an experiment I copied the menu.inc from alpha7 over the one from the current CVS
and the menu reappeared. Same with the equivalent block from the menu_block module.