If your query var tags
is registered properly then the following code snippet will definitely work, I’ve tested it. It’s a modified version of your code snippet. Please make sure to add the code to your functions.php
file.
function prefix_custom_title($title_parts) {
global $wp_query;
if (isset($wp_query->query_vars['tags'])) {
$title_parts['title'] = $wp_query->query_vars['tags'];
}
return $title_parts;
}
add_filter( 'document_title_parts', 'prefix_custom_title' );