Hello,
Thanks for contacting Rank Math support. We apologize for any inconvenience this might have caused you.
Please share your sitemap URL so we can check.
Meanwhile, please follow the steps below to see if that works for you.
#1. Flush the Sitemap cache by following this video screencast:
https://i.rankmath.com/pipRDp
#2. Exclude the Sitemap files of the Rank Math plugin from caching. The cache could be via a plugin or from the server. For plugins or Cloudflare, please follow this article:
https://rankmath.com/kb/exclude-sitemaps-from-caching/
#3. If the above steps don’t seem to work, kindly apply the following filter code to your site.
add_filter( 'rank_math/sitemap/enable_caching', '__return_false');
Here’s how you can add filter/hook to your WordPress site: https://rankmath.com/kb/wordpress-hooks-actions-filters/
Once done, please clear your website cache, including server-side cache, and check the sitemap again.
If the issue persists, please edit the first post on this ticket and include your WordPress & FTP logins in the designated Sensitive Data section.
Please do take a complete backup of your website before sharing the information with us.

It is completely secure, and only our support staff has access to that section. If you want, you can use the plugin below to generate a temporary login URL to your website and share that with us instead:
https://wordpress.org/plugins/temporary-login-without-password/
You can use the above plugin in conjunction with the WP Security Audit Log to monitor what changes our staff might make on your website (if any): https://wordpress.org/plugins/wp-security-audit-log/
Please do take a complete backup of your website before sharing the information with us.
We really look forward to helping you.