Forces Yoast to use the display name directly for the queried object so the proper coauthor name is used as the page title.

yoast-seo-with-coauthors-plus.php

<?php // only copy if needed!

/**
 * Fixes page titles for co-author archives.
 *
 * @param string[] $replacement Yoast title settings replacement data
 * @return string[] updated data
 */
function br_fix_author_archive_title( $replacement ) {

	if ( is_author() && isset( $replacement['%%name%%'] ) ) {
		$replacement['%%name%%'] = get_queried_object()->display_name;
	}

	return $replacement;
}
add_filter( 'wpseo_replacements', 'br_fix_author_archive_title' );