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Menu Attributes provides hook_menu_attributes_info to allow additional Form API elements to be included for saving within menu item options. This includes text_format Form API elements. However, text_format saves its value/format combination in the format
$text_format_value = array( 'value' => $text_value, 'format' => $text_format, );
_menu_attributes_form_alter() does not account for this structure when setting the #default_value of its form elements, and so the saved text_format value/format combination is not loaded correctly into the form.
The attached patch has handling for the default value of text_format form elements.