If you go to the applications configure dialog list i.e. https://i.imgur.com/g1jvmGW.png
Clicking on the list doesn't seem to focus it, thus pressing the up/down arrows doesn't change the active application on the list, which is what i would expect to happen.
If you "tab focus" into it, it will work, so it may seem is more a focus issue than a keyboard handling issue
Here's a patch: https://phabricator.kde.org/D25309
Git commit 659fe0a76f19b60021733c4a74e451d4318a8359 by Nate Graham.
Committed on 18/11/2019 at 18:40.
Pushed by ngraham into branch 'Plasma/5.17'.
[Notifications KCM] Fix keyboard navigation in apps list
Learned how to do this from @guoyunhe in D24798!
1. Click on list item -> list gets focus, item is shown
2. Use up and down arrow keys -> items are shown
3. Click on search field and tab into list -> you can select things with the keyboard
4. Search for something and click on or tab into apps list -> the correct item is shown
Reviewers: broulik, #plasma
Subscribers: plasma-devel, guoyunhe
Differential Revision: https://phabricator.kde.org/D25309
M +9 -3 kcms/notifications/package/contents/ui/SourcesPage.qml