Nobody here understood your concern but I did because it is happening to me as well. I can see all the pages of my blog are indexed in Google but have been removed from Sitemap in GSC. I tried to resolve this problem but failed. I removed the sitemap and resubmitted it, I removed the plugin that I was using to generate the sitemap in XML format and reinstall it, I tried other plugins as well. I waited for a month but nothing worked for me. It looks like we can not control it, it's Google who has full control of it. Prolly it's a sign that our website is hit by Google updates.