Hello,
Thank you for contacting the support.
Indexed, Not Submitted in Sitemap is not an issue. It only means that Google has already indexed the URL but not through the sitemap.
Google may be crawling and indexing the posts through other means like the internal links on your website and not through the sitemaps, as a result, the crawl budget for your website is exhausted, and Google is unable to discover the URLs through your sitemap.
As long as the posts/pages are getting crawled and indexed, there’s nothing to be worried about.
Hope this helps and please feel free to let us know if you have any questions.
Thank you.
