I think you’re looking for the acf/fields/flexible_content/layout_title
filter. Assuming the name on that Title dropdown is just ‘title’ I think this should do it?
add_filter('acf/fields/flexible_content/layout_title', function($title) {
$ret = $title;
if ($custom_title = get_sub_field('title')) {
$ret = sprintf('<strong>%s</strong> <em style="font-size: 80%; opacity: 0.5">%s</em>', $custom_title, $title);
}
return $ret;
});