Hello @khanhvo,
Thank you for contacting Rank Math support, and sorry for any inconvenience that might have been caused due to that.
Please follow these steps to flush your sitemap cache:
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 in your caching plugin. 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. Disable the transient cache for the sitemap by applying this filter. This will make sure that your XML sitemap always reflects the latest changes on your site:
/**
* Filter if XML sitemap transient cache is enabled.
*
* @param boolean $unsigned Enable cache or not, defaults to true
*/
add_filter( 'rank_math/sitemap/enable_caching', '__return_false');
If you’re not sure how to add this code, you can follow this guide:
https://rankmath.com/kb/wordpress-hooks-actions-filters/
Hope that helps.