Yoast SEO: 1.1.9
Since upgrading to Yoast, I've noticed that my sitemap.xml file has disappeared. Google webmaster brought this to my attention.
Yoast SEO says my sitemap is called sitemap_index.xml but a) this file does not exist in the root and b) even if it did, Google is looking for sitemap.xml not sitemap_index.xml
I copied an older version of sitemap.xml (off my local machine) into the root and Google Webmaster found it straight away. I am sure this older version of sitemap.xml was created by Yoast SEO.