It could be an issue related to W3TC. Somehow, it changes Content-Type text/xml to text/html in some cases.
$ curl -i https://rockandblog.news/sitemap_index.xml
HTTP/1.1 200 OK
.....
Content-Type: text/html; charset=UTF-8
<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//rockandblog.news/wp-content/plugins/wordpress-seo/css/main-sitemap.xsl"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
.....
<!--
Performance optimized by W3 Total Cache. Learn more: https://www.w3-edge.com/products/
.....
Served from: rockandblog.news @ 2019-12-12 08:21:25 by W3 Total Cache
-->
Thanks I have deleted W3C and now It seems to be in text/xml format but I still only see 2 urls in search console 🙁
curl -i https://rockandblog.news/sitemap_index.xml
HTTP/1.1 200 OK
Date: Thu, 12 Dec 2019 19:58:39 GMT
Server: Apache/2.4.34 (Ubuntu)
X-Robots-Tag: noindex, follow
Vary: Accept-Encoding
Content-Length: 1048
Content-Type: text/xml; charset=UTF-8
<?xml version=”1.0″ encoding=”UTF-8″?><?xml-stylesheet type=”text/xsl” href=”//rockandblog.news/wp-content/plugins/wordpress-seo/css/main-sitemap.xsl”?>
<sitemapindex xmlns=”http://www.sitemaps.org/schemas/sitemap/0.9″>
<sitemap>
<loc>https://rockandblog.news/post-sitemap.xml</loc>
<lastmod>2019-12-12T17:07:51+00:00</lastmod>
</sitemap>
<sitemap>
<loc>https://rockandblog.news/page-sitemap.xml</loc>
<lastmod>2019-12-12T17:23:59+00:00</lastmod>
</sitemap>
<sitemap>
<loc>https://rockandblog.news/tdb_templates-sitemap.xml</loc>
<lastmod>2019-12-12T17:23:45+00:00</lastmod>
</sitemap>
<sitemap>
<loc>https://rockandblog.news/category-sitemap.xml</loc>
<lastmod>2019-12-12T17:07:51+00:00</lastmod>
</sitemap>
<sitemap>
<loc>https://rockandblog.news/post_tag-sitemap.xml</loc>
<lastmod>2019-12-12T17:07:51+00:00</lastmod>
</sitemap>
<sitemap>
<loc>https://rockandblog.news/author-sitemap.xml</loc>
<lastmod>2019-12-03T17:36:21+00:00</lastmod>
</sitemap>
</sitemapindex>
I think that you should wait about 24-48 hours to google refresh sitemaps cache after you re-submit sitemap index. I see that you changed plugin, but I’m not sure that will speed up process.