On a London instance, navigate to "Service Portal=>Knowledge" and then search for a keyword (for example "email") in the searchbox near the breadcrumb, system would display the matching KB articles.
Now, try again searching for a new keyword ( for example "test"), you will see the typeahead search populates the list of articles on the search field, when you click enter, system doesn't show the results in the main search page as shown in the below screenshot.
London release onwards.
On London release, Page route map for Search page has been introduced and it should be enabled so that search page would work as expected.
On the affected instance, search page route map was de-activated as shown in the below screenshot.
Activate the search page route map.