<?php
/**
 * @param $title
 * @return array
 */
function archive_titles($title): array
{
	global $post;

	$type = $post->post_type;

	$types = [
		'custom_post_type' => 'Custom title',
	];

	if (is_archive() && isset($types[$type])) {
		$title['title'] = $types[$type];
	}

	return $title;
}

add_filter('document_title_parts', 'archive_titles');