This part of code in Zend_Dojo_Form_Element_ComboBox line 152

    public function setAutocomplete($flag)
    {
        $this->setDijitParam('autocomplete', (bool) $flag);
        return $this;
    }

in conjunction with

Zend_Dojo_View_Helper_Dojo::setUseDeclarative();

when trying to set autocomplete feature to 'false' provides a dijit markup with autocompelete="" property which actualy means no value and autocomplete stays on (as it is by default in dojo). Same goes to Dojo_Form_Element_FilteringSelect as it derives from combobox.

Suggestion: set property as is, without type casting.