On git master frameworks/plasma, updated yesterday. When in kde system settings (but not when invoked as a dedicated single kcm view via kcmshell5), viewing a rule in the window rules kcm, there's no way to get back to the rules list. One must click on a different kcm, then back to rules, in ordered to get back to the window list. In the dedicated rules kcm view (not in kde systemsettings), the header-bar at the top has a breadcrumbs path with a left-arrow that can be clicked to get back to the rules list, or the window rules section of the breadcrumbs can be clicked to do the same thing. But that header-bar doesn't appear in kde systemsettings view, so there's no breadcrumbs and no left-arrow or windows-rules section in it to be clicked to get back. =:^(
This was a temporary bug that has not been fixed in today's git master. :) They don't call it bleeding edge for nuthin'!