Using parameters of activeSceneChanged?

I was trying to use SceneManager.activeSceneChanged to store the buildIndex of the current and previous scene, using the information about it I found in another question (since the documentation is incomplete) like this:

 void Awake () {
         DontDestroyOnLoad(this);
         SceneManager.activeSceneChanged += MyMethod;
     }
 
     void MyMethod(Scene previousScene, Scene newScene)
     {
         previousSceneIndex = previousScene.buildIndex;
         currentSceneIndex = newScene.buildIndex;
     }

the currentSceneIndex is always correct and updates with every scene change, but the value for previousSceneIndex starts with -1 and stays that way.

I solved my issue by just ignoring the parameters and adjusting my values like so:

 previousSceneIndex = currentSceneIndex;
 currentSceneIndex = SceneManager.GetActiveScene ().buildIndex;

but I still would like to know why the other way didn't work, or what the correct use of activeSceneChanged looks like. Is that first parameter not meant to be accessed?