Custom app not reading prefs since upgrade to Sonoma

Since upgrading to macOS Sonoma, having an issue with app prefs with a custom app I have been working on. The prefs are being read from some location I cannot determine, yet prefs are being written to the usual Library->Containers->[app]->...->Preferences. If I edit the prefs plist in Preferences, the change is not reflected when I restart the app; but, if the app makes changes to prefs, the prefs plist in Preferences is overwritten.