What triggers the issue?
This issue reports pages declared as alternate language versions that don't list themselves in their hreflang annotations.
Why is important?
The rel="alternate" hreflang="x" link attribute helps manage multilingual websites by allowing you to specify language- and region-specific versions of a page. Based on this information, the search engine can serve the needed version to the users based on their language (and alternatively location).
All language variations of the same page need to be interlinked via hreflang annotations. In plain English, if you have About Us page in 3 different languages, each of these 3 pages need to link to the other 2 versions as well as have a self-referencing hreflang. If the latter is missing, it can confuse the search engines and the page version might be left out.
How to fix it?
You can manage your hreflang annotations via HTML tag:
<link rel="alternate" hreflang="lang_code" href="url_of_page" />
as well as via HTTP headers:
Link: <http://example.com/file.pdf>; rel="alternate"; hreflang="en",
<http://de-ch.example.com/file.pdf>; rel="alternate"; hreflang="de-ch",
<http://de.example.com/file.pdf>; rel="alternate"; hreflang="de"
Whichever method you are using you need to make sure each language- or region-specific page has a self-referencing annotation. For example, a Spanish version of your page must have rel="alternate" hreflang="es" annotation with a link to itself.