When I build your site locally, this is what is rendered in public/sitemap.xml
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<sitemap>
<loc>https://www.casanospam.com/it/sitemap.xml</loc>
</sitemap>
<sitemap>
<loc>https://www.casanospam.com/en/sitemap.xml</loc>
</sitemap>
</sitemapindex>
The problem is in your Netlify configuration:
HUGO_BASEURL = "/"