Why is my scene variable losing its value?

So I have a script that assigns the last scene that was loaded (if there was one) to a variable called "lastLoadedScene," which happens after a scene is loaded. All this is to determine when to switch what audioclip is playing in the background depending on the scene. But whenever I try to access the lastLoadedScene after it's already been assigned a value the last time a scene was loaded, it will return null.

 public class MusicManager : MonoBehaviour {
     public AudioClip[] levelMusicChangeArray;
     private AudioClip thisLevelMusic, previousLevelMusic;
     private AudioSource audioSource;
     private Scene lastLoadedScene;
 
     private void OnEnable () {
         SceneManager.sceneLoaded += OnSceneLoaded;
     }
     private void OnDisable () {
         SceneManager.sceneLoaded -= OnSceneLoaded;
     }
 
     void OnSceneLoaded (Scene scene, LoadSceneMode loadSceneMode) {
         thisLevelMusic = levelMusicChangeArray [scene.buildIndex];
         if (lastLoadedScene.IsValid () == true) { // This condition if never true no matter how many scenes are loaded, so previousLevelMusic never gets a value
             previousLevelMusic = levelMusicChangeArray [lastLoadedScene.buildIndex];
         }
             
         if (thisLevelMusic && thisLevelMusic != previousLevelMusic) {
             audioSource.clip = thisLevelMusic;
             audioSource.loop = true;
             audioSource.Play ();
         }
         
         lastLoadedScene = scene; // This is where I assign lastLoadedScene to the scene that was just loaded. However, whenever the game comes back to this method to load a level, lastLoadedScene doesn't have a value anymore
     }
 
     void Awake () {
         DontDestroyOnLoad (gameObject);
     }
 
     void Start () {
         audioSource = GetComponent<AudioSource> ();
     }
 }