Editor changes lost when entering Playmode

I'm certain I'm misunderstanding how Unity's serialization works but when i enter play mode all changes made through my custom editor are instantly reverted to the last value before the editor made its changes. i.e my editor changes a public int from a GameObject from 50 to 72 when entering play mode the value becomes 50 again, if i change the value manually the value will remain the correct value(72). Just to be clear the editor works exactly as expected.

cutting my code back to the relevant points i have the following setup

 [CustomEditor (typeof(WayPoint))]
     public class WaypointController : Editor
     {
         foreach (GameObject o in SelectedWaypoints)
         {
             o.GetComponent<WayPoint>().Priority = 72;
             EditorUtility.SetDirty(o);
         }
     }

 [System.Serializable]
 public class WayPoint : MonoBehaviour
 {
 [SerializeField]
     public int
         Priority;
 }

my understanding is that atomic types such as int shouldn't need to be tagged in this manner but I simply can't get changes made by my editor class to be serialized.

Thanks in advance for any and all help.