open the sddm_kcm in the system settings select some sddm theme click "remove theme" button (sddm kcm asks for your password) enter your password to confirm theme removing close the system settings Apply/discard prompt shows up click "discard" button reopen sddm_kcm: the sddm theme really was removed despite you clicked in "discard" button.
Not reproducible with the new SDDM KCM on neon unstable. Operating System: KDE neon Unstable Edition KDE Plasma Version: 5.20.80 KDE Frameworks Version: 5.79.0 Qt Version: 5.15.2