The issue is that an article does not appear in search results. When searched for this article, it only returns articles have that this article number mentioned in it's contents.
There could be plenty of reasons why a certain knowledge article is not searchable. One of the reasons is that the article is no longer valid.
The Knowledge form has a "Valid To" field which is the date that knowledge article expires. Articles do not appear in search results after the valid to date, or if the valid to date is empty.
To fix the issue, add a future date in Valid To field and article will start to appear in search results.