Hello,
Thank you for contacting Rank Math, and sorry for any inconvenience caused.
The 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 nothing to worry about as long as the posts/pages are getting crawled and indexed.
Hope that helps.
Thank 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.
