Can a WordPress loopback failure still break scheduled posts when a real server cron is running?

It can, but only if the server cron depends on the same failing self-request or isn't actually running wp-cron.php successfully. A system cron that runs the local PHP wp-cron.php script on schedule can keep scheduled posts working even when WordPress Site Health reports a loopback problem. But a cron job that calls https://your-site/wp-cron.php can hit the same firewall, auth, DNS, or SSL issue as the loopback test. I'd check which command the host is using, make sure it targets the right WordPress install, and review its exit/error logs. Also set DISABLE_WP_CRON to true when using a system cron, so page loads aren't trying to trigger it too. If posts still get missed, check the scheduled event and any plugin or server errors; the loopback warning may still matter for plugin background tasks.