Thanks for your quick response.I am using OOTB sitemap which AEM Cloud provides and this issue is resolved now , as few browser hide this specific tag  <?xml version="1.0" encoding="UTF-8"?> but in network response I am able to see that generated sitemap is encoded with UTF-8. 

<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">    <sitemap>       <loc>http://www.example.com/sitemap1.xml.gz</loc>       <lastmod>2004-10-01T18:23:17+00:00</lastmod>    </sitemap>    <sitemap>       <loc>http://www.example.com/sitemap2.xml.gz</loc>       <lastmod>2005-01-...