Checked{NO ERRORS}Check if your sitemap is working, submitted in GSC, and have no errors.
Yes (many times)Have you resubmitted your sitemap?
If everything is working then look into the details.. Find the "indexed but not submitted" pages in your submitted sitemap files, match the URLs too. Because It is possible that those pages are no more there in your submitted sitemap or the URLs have been changed but old URLs are still working so they are indexed but not present in the sitemap.Checked{NO ERRORS}
I checked my sitemap and all url's available there. But still they are showing "index but not submitted in sitemap"If everything is working then look into the details.. Find the "indexed but not submitted" pages in your submitted sitemap files, match the URLs too. Because It is possible that those pages are no more there in your submitted sitemap or the URLs have been changed but old URLs are still working so they are indexed but not present in the sitemap.
Actually a bunch of url's is deindex from google and again indexed ,but they lost their position and shifted to "index and not submitted in sitemap"If you've submitted your sitemap, then investigate what URL Google’s reporting to be missing and verify whether these URL need to be indexed.