Correct the cookie domain when a custom domain is mapped to the site.

correct-cookie-domain.php

<?php

/**
 * Correct the cookie domain when a custom domain is mapped to the site.
 * 
 * @see https://blog.handbuilt.co/2016/07/07/fixing-cookie_domain-for-mapped-domains-on-multisite/
 */
add_action( 'muplugins_loaded', function(){
	global $current_blog, $current_site;
	if ( false === stripos( $current_blog->domain, $current_site->cookie_domain ) ) {
		$current_site->cookie_domain = $current_blog->domain;
	}
});