Despite the fact that the XML sitemaps are correctly generated and not empty, you may see 0 links when opening a formatted sitemap page. This problem is caused by the Admin Tools plugin if the option to force the website running on https is enabled, so that it replaces every links encountered in the Joomla! output with the https protocol version.
Indeed the correct link for the namespace of an XML sitemap document is:
and if it's replaced with the https version it won't be valid causing this side effect:
The problem is caused by the setting named 'Convert all links to HTTPS when site is accessed over SSL' of Admin Tools. Ensure that it's set to 'No' to fix the problem.