Applications for which the default KWin titlebar and frame are disabled (through Window Rules) show by default on all desktops, and there is no way to change this behaviour besides by adding back the titlebar. However, for a web browser (such as Chromium or Vivaldi, my current browser), with tabs at the top, this appearance does not make sense. There should be a setting that allows users to hide the titlebar and frame of a particular application while still maintaining KWin handling so as to let these windows be restricted to the activity on which they were launched, like all other KWin-handled apps. Users would still be able to change the activity on which the window is displayed using alt-F3 to bring up the menu.
*** This bug has been marked as a duplicate of bug 274931 ***