It happened a couple of times that I (and customers) did not find a desired article using the search function in Guide, although I used the title of the article as a search word.
Zendesk told me the following:
1) An article is connected to a category
2) If leveraging a search from within articles, search is conducted within the category the article is connected to (by design of Guide).
3) If conducting a search from the landing page, search operates on the whole Help Center.
This search separation between searching in one category and within the whole Guide is is intended and not seen as a bug.
This search logic does not make sense to me. My solution will be to disable the search bar within the articles, such that the customer can only launch searches from the landing page.