Here an Issue which should be added to the “Known issues and solutions” list.
Issue: No search results in application launcher when using multiple kickoff instances.
Solution 1: Manually Downgrading the Kickoff Applet until the Bug is fixed Upstream.
Solution 2: Only have one Kickoff Applet, by removing the other ones.
Upstream KDE Bug: 443131 – When there's more than one Kickoff, clicking on the old one causes search to be broken until you search with the new one
(Basically they completly rewrote the codebase of the kickoff applet, which seems to only create a single search instance, which they try to bind to multiple kickoff applets. However this seems to result in some NULL exceptions, as it fails to force focus on the search object.)
Workaround Tutorial: No search results in application launcher - #6 by alkl