Found the problem: Syncdaemon was changed to use the new com.ubuntu.sso.CredentialsManagement DBus interface, but the gnome-settings-daemon plugin still uses the deprecated com.ubuntu.sso.ApplicationCredentials interface.
So, when Syncdaemon requests the credentials using find_credentials in the first interface, the CredentialsFound signal is only thrown from that same interface, and not from the deprecated one (and viceversa).
And this is why the gsd-plugin does not catch that signal and stores the bookmark.
Found the problem: Syncdaemon was changed to use the new com.ubuntu. sso.Credentials Management DBus interface, but the gnome-settings- daemon plugin still uses the deprecated com.ubuntu. sso.Application Credentials interface.
So, when Syncdaemon requests the credentials using find_credentials in the first interface, the CredentialsFound signal is only thrown from that same interface, and not from the deprecated one (and viceversa).
And this is why the gsd-plugin does not catch that signal and stores the bookmark.
I'm working on a gsd-plugin branch to fix this.