Since the setting to draw the separator only affects the active window, the setting should be renamed, e.g. to "Draw separator between active window and titlebar".
Git commit 1a9dc381c8c146d26106be8715c9c561cd2ca04e by Nate Graham. Committed on 01/05/2020 at 13:35. Pushed by ngraham into branch 'master'. Phrase active window titlebar separator option more accurately FIXED-IN: 5.19.0 M +1 -1 kdecoration/config/ui/breezeconfigurationui.ui https://commits.kde.org/breeze/1a9dc381c8c146d26106be8715c9c561cd2ca04e
Thanks!