Drupal 6 form textfield #default_value not working

Surprisingly textfield #default_value is not working in views exposed form. I have enabled search field exposed form and using hook_form_alter I am setting its #default_value property. But while rendering it doesn't show default value. However I found that inside views_handler_filter_search.inc when I change form type textfield to textarea as shown below:

function exposed_form(&$form, &$form_state) {
    if (isset($this->options['expose']['identifier'])) {
      $key = $this->options['expose']['identifier'];
      $form[$key] = array(
        '#type' => 'textfield',
        '#size' => 15,
        '#default_value' => $this->value,
        '#attributes' => array('title' => t('Enter the terms you wish to search for.')),
      );
    }
  }

TO

  function exposed_form(&$form, &$form_state) {
    if (isset($this->options['expose']['identifier'])) {
      $key = $this->options['expose']['identifier'];
      $form[$key] = array(
        '#type' => 'textarea',
        '#size' => 15,
        '#default_value' => t('Search'),
        '#attributes' => array('title' => t('Enter the terms you wish to search for.')),
      );
    }
  }

default value start appearing.

Could you please help me out to rectify this issue.

EDIT:

Along with Search textfield I have taxonomy select menu filter to narrow search results. When I set #after_built function and inside function when I printed form variable, found that #post is by default set as shown below:

Array
(
    [#type] => textfield
    [#size] => 15
    [#default_value] => Search
    [#attributes] => Array
        (
            [title] => Voer hier een berichtnummer of tekst in..
            [class] => inputbox
        )

    [#after_build] => Array
        (
            [0] => after_built_search
        )

    [#post] => Array
        (
            [term] => All
            [form_build_id] => form-d08424eee7a527d39d7569acb028599f
            [form_id] => views_exposed_form
        )

    [#programmed] => 
    [#tree] => 
    [#parents] => Array
        (
            [0] => keys
        )

    [#array_parents] => Array
        (
            [0] => keys
        )

    [#weight] => 0
    [#processed] => 1
    [#description] => 
    [#required] => 
    [#input] => 1
    [#maxlength] => 128
    [#autocomplete_path] => 
    [#process] => Array
        (
            [0] => form_expand_ahah
        )

    [#name] => keys
    [#id] => edit-keys
    [#value] => 
    [#needs_validation] => 1
    [#defaults_loaded] => 1
    [#sorted] => 1
)