I am using 1.3.7 and noticed this as well. To fix this you can also replace the for loop around line 849 in admin/categories.php with this. This will just use the standard HTML editor for all meta tags, so you don't have to switch editors manually to edit the tags.
for ($i = 0, $n = sizeof($languages); $i < $n; $i++) {
$category_inputs_string_metatags_description .= '<br />' . zen_image(DIR_WS_CATALOG_LANGUAGES . $languages[$i]['directory'] . '/images/' . $languages[$i]['image'], $languages[$i]['name']) . ' ' ;
/gpg mod, unable to load fckeditor here. Could fix this, but why enable fckeditor here, for meta tags we don't need
any special HTML editor/
/* if ($_SESSION['html_editor_preference_status']=='FCKEDITOR') {
$oFCKeditor = new FCKeditor('metatags_description[' . $languages[$i]['id'] . ']') ;
$oFCKeditor->Value = zen_get_category_metatags_description($cInfo->categories_id, $languages[$i]['id']);
$oFCKeditor->Width = '97%' ;
$oFCKeditor->Height = '200' ;
// $oFCKeditor->Config['ToolbarLocation'] = 'Out:xToolbar' ;
// $oFCKeditor->Create() ;
$output = $oFCKeditor->CreateHtml() ;
$category_inputs_string_metatags_description .= '<br />' . $output;
// $category_inputs_string_metatags_description .= '<IFRAME src= "' . DIR_WS_CATALOG . 'FCKeditor/fckeditor.html?FieldName=metatags_description[' . $languages[$i]['id'] . ']&Upload=false&Browse=false&Toolbar=Short" width="97%" height="200" frameborder="no" scrolling="yes"></IFRAME>';
// $category_inputs_string_metatags_description .= '<INPUT type="hidden" name="metatags_description[' . $languages[$i]['id'] . ']" ' . 'value=' . "'" . zen_get_category_metatags_description($cInfo->categories_id, $languages[$i]['id']) . "'>";
} else {/
$category_inputs_string_metatags_description .= zen_draw_textarea_field('metatags_description[' . $languages[$i]['id'] . ']', 'soft', '100%', '20', zen_get_category_metatags_description($cInfo->categories_id, $languages[$i]['id']));
/}*/
}