java.util.logging Formatters will traditionally catch MissingResourceException when invoking ResourceBundle::getString, and use the original key string as message if a MissingResourceException is thrown.

The SimpleConsoleLogger however does not have this logic and will throw a MissingResourceException in that case. For the sake of uniformity it would be better if SimpleConsoleLogger behaved in the same way that java.util.logging does.
Also System.Logger::log doesn't have any @throws MissingResourceException.