As for small and new sites, that's pretty common and has nothing to do with Google Sitemaps. When it comes to established Web sites disappearing partly or even completely, that's another story.
There is an underestimated and often overseen risk assigned to sitemap submissions. Webmasters who have made use of sitemap tools generating the sitemaps from the web server's file system, may have submitted 'unintended spider food' to Google, and quickly triggered a spam filter.
At least with important sites, it's a must to double-check the generated sitemaps before they get submitted. Sitemap generators may dig out unlinked and outdated junk, for example spider trap links pages and doorway pages from the last century.
More info here.
Tags: Search Engine Optimization (SEO) Google Sitemaps Google
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