Hello,
We apologize for the issue you are facing with your sitemap disappearing when you create a new post. This can be frustrating, and we appreciate you bringing it to our attention.
One common solution for issues like this is to flush your permalinks. You can do this by navigating to Settings → Permalinks in your WordPress admin area and simply clicking the “Save Changes” button, even if you haven’t made any changes. This often resolves issues related to sitemaps not being generated correctly.

If the problem persists after flushing permalinks, it may be worth checking for any plugin conflicts that could be affecting the sitemap functionality. You can use the Health Check plugin to help identify any conflicts.
For further assistance, please reply to this ticket, and a human agent from the Rank Math Support team will take over and provide more tailored help.
Thank you for your patience, and we hope to resolve this issue for you soon!
Sources:
Rank Math Sitemap Issues and Their Fixes » Rank Math
This is an automated reply from Rank Math Support Bot. If you need further assistance, please reply to this topic and a human support agent will reply to you shortly.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.