Only 1st subform is validated

Digging a bit more I see where it falls:

On the first form, we have all the required values in $form_state['input'], so in form_builder()

    if ($form_state['programmed'] || (!empty($form_state['input']) && (isset($form_state['input']['form_id']) && ($form_state['input']['form_id'] == $form_id)))) {
      $form_state['process_input'] = TRUE;
    }

is executed.

However clicking on validate/ submit on the second subform doesn't populate $form_state['input'] properly (see images).

And now I see why the $_POST values are different. The 2nd subform has <form> tag, while the 1st one doesn't. (see image)

Ha, I think I got it ;) -- change the subform's #theme_wrapper to container instead of form.

Better patch, it seems that this was supposed to work already, however in after_build we add #pre_render to $form instead of $element, resulting in never calling the pre-render function.