The CMake variables that define where the config folder and files live take into account the install prefix rather than being hardcoded to /etc. As a result, if you compile your own plasma to, say, ~/kde/usr, then using the sync feature will write a root-owned file to ~/kde/usr/etc/sddm.conf.d/kde_settings.conf, which causes the feature to not work, since SDDM only looks for the file at /etc/sddm.conf.d/kde_settings.conf.
See https://invent.kde.org/plasma/sddm-kcm/-/commit/c879e4fa438f4cb029f1e73784489869b0d48314
Yeah, that commit makes sense for the purpose of reading the settings, but not writing them, now that they're written too. :)
Git commit 5585b084c56c4b347c7b575fd7a403e3138fb9fa by Nate Graham, on behalf of David Edmundson. Committed on 09/08/2022 at 22:58. Pushed by ngraham into branch 'master'. Avoid install prefix in config path lookup CMAKE_INSTALL_FULL_SYSCONFDIR has unexpected behaviour when used with prefixes. > read-only single-machine data (etc, or /etc if CMAKE_INSTALL_PREFIX is /usr) This is typically because prompting for root every install would be annoying. Our use case here is not to install files but to refer to a file in a known location once installed, so we do not want this behaviour. FIXED-IN: 5.26 M +2 -2 src/CMakeLists.txt https://invent.kde.org/plasma/sddm-kcm/commit/5585b084c56c4b347c7b575fd7a403e3138fb9fa