Remove the WordPress front matter slug from the author url.

wordpress_author_url.php

<?php 
/**
 * Modify Author links go to the right page
 */
function dk_author_link( $link, $author_id ) {
	$link = str_replace( 'FRONT_URL_PATH/', '', $link );
	return $link;
}
add_filter( 'author_link', 'dk_author_link', 10, 2 );

/**
 * Custom rewrite rules to make sure Author links go to the right page
 */
function dk_author_url( $author_rules ) {
	foreach ($author_rules as $old_key => $val) {
		$new_key = str_replace( 'FRONT_URL_PATH/', '', $old_key );
		$author_rules[$new_key] = $val;
		unset( $author_rules[$old_key] );
	}
	return $author_rules;
}
add_filter( 'author_rewrite_rules', 'dk_author_url' );