Default Value and AHAH

There seems to be a prolem with form building when Javascript is enabled on multivalue fields and "Add another item":

I use form_alter to provide default values for my custom cck field, consisting of 3 textfields.
It works nice for the 2 default items. they get filled with the values I set in form_alter.

- If JS is disabled in the browser, all further items are populated, too.
- If JS is enabled, I can do what I want, the form doesn't set the default values.

I tried form_alter and after_build

fmi:
setting after_build on form_alter for javascript call ($form_id = content_add_more_js) like
$form['#after_build'][] = 'cck_inputs_checkbox_checkbox_process_afterbuild';

doesn't trigger it.
But it gets triggered when registered on the standard form_alter call.

code runs like this:

on "create node":
standard form_alter (with registering after_build )
after_build

on "add another item":
after_build
form_alter javascript
after_build

/fmi

what I found out is this:

after this call to form_builder in content_add_more_js:
$form = form_builder($_POST['form_id'], $form, $form_state);

the form is populated inconsistently for default values:

......
[1] => Array
                        (
                            [#type] => cck_inputs_checkbox_checkbox
                            [#default_value] => Array
                                (
                                    [checkbox_label] => Seat 1
                                    [checkbox_name] => checkbox_name_1
                                    [checkbox_value] => 1
                                )
                            ........
                           [checkbox_label] => Array
                                (
                                    [#title] => Checkbox label
                                    [#type] => textfield
                                    [#default_value] => Seat 1
                          ........
 [2] => Array
                        (
                            [#type] => cck_inputs_checkbox_checkbox
                            [#default_value] => Array
                                (
                                    [checkbox_label] => Seat 3
                                    [checkbox_name] => checkbox_name_3
                                    [checkbox_value] => 3
                                )
                             ......
                            [checkbox_value] => Array
                                (
                                    [#title] => Checkbox value
                                    [#type] => textfield
                                    [#default_value] => 
                             ..........

on the newly added item [2] the default value is not set. only for the widget.