The reason it works is that serialization process for Enum‘s is different from serialization process for other classes. From the official documentation:
1.12 Serialization of Enum Constants
Enum constants are serialized differently than ordinary serializable or externalizable objects. The serialized form of an enum constant consists solely of its name; field values of the constant are not present in the form. To serialize an enum constant, ObjectOutputStream writes the value returned by the enum constant’s name method. To deserialize an enum constant, ObjectInputStream reads the constant name from the stream; the deserialized constant is then obtained by calling the java.lang.Enum.valueOf method, passing the constant’s enum type along with the received constant name as arguments. Like other serializable or externalizable objects, enum constants can function as the targets of back references appearing subsequently in the serialization stream.
That means, all your custom fields won’t be serialized. In your case everything works well because your application process is still running and you are getting the same Enum instance that you passed to savedInstanceState.putSerializable.
But imagine a situation where your app get killed because Android has not enough memory. The next time user opens the app you will get a new Enum instance and all custom fields will have been lost and reinitialized by the constructor. Thus, mutable fields in an enum are always effectively transient.