Hello,
Thank you for contacting Rank Math.
Indexed, Not Submitted in Sitemap is not an issue, it only means that Google has already indexed the posts but not through the sitemap.
It is possible that Google is crawling your website and indexing the posts through the internal links on your website and not through the sitemap, and as a result, the crawl budget for your website is exhausted and Google is not able to discover the URLs through your sitemap.
There is no need to worry about this warning as the posts/pages are getting indexed.
You should exclude the sitemaps from caching(if you haven’t already) then remove the sitemaps from the Search Console and add the sitemap_index.xml again.
Here is a guide you can follow to exclude the sitemaps from caching:
https://rankmath.com/kb/exclude-sitemaps-from-caching/
I hope this helps. Looking forward to helping you.
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.
