Not sure if it is true for GNOME Flashback, but regular GNOME uses gnome-settings-daemon to manage keyboard configuration. And its keyboard manager uses GSettings to store the keyboard configuration, only using the xkb config for initial set up:
- plugins/keyboard/gsd-keyboard-manager.c · 13fdc5b74620b414ffe35f7a0ef5aab4686b3b2d · GNOME / gnome-settings-daemon · GitLab
- plugins/keyboard/gsd-keyboard-manager.c · 13fdc5b74620b414ffe35f7a0ef5aab4686b3b2d · GNOME / gnome-settings-daemon · GitLab
- plugins/keyboard/gsd-keyboard-manager.c · 13fdc5b74620b414ffe35f7a0ef5aab4686b3b2d · GNOME / gnome-settings-daemon · GitLab
You can clean up the settings using
gsettings reset org.gnome.desktop.input-sources xkb-options
gsettings reset org.gnome.desktop.input-sources sources
and the system defaults should be picked up after re-login.