Canonical URL: Always add a trailing slash to the canonical URL.

canonical.php

<?php

namespace WPS\canonical;

add_filter( 'get_canonical_url', 'WPS\canonical\get_canonical_url' );
/**
 * Add a trailing slash to the end of the path.
 *
 * @param string $canonical_url Canonical URL.
 * @return string Maybe modified URL.
 */
function get_canonical_url( $canonical_url ) {
	if ( "/" === substr( $canonical_url, -1 ) ) {
		return $canonical_url;
	}
	$url = parse_url( $canonical_url );
	$surl = $url['scheme'] . "://" . $url['host'] . trailingslashit( $url['path'] ) . '?' . $url['query'] . '#' . $url['fragment'];

	return $surl;
}