Why are the fields of my SerializedObject null when I set another field to its default value?

I am trying to learn more about how to write play mode tests in Unity3D and I am facing a peculiar behaviour in one of my tests. I am using SerializedObject to set the values of protected fields of my ScriptableObject class CurvedMovement but whenever I attempt to set the _speed field to the same value as its default (50) then the other fields, _xCurve and _yCurve, are null. I've tried testing with loads of different values and even tried changing the default value in the abstract class Movement but every time the test sets the _speed field to exactly the same value as its default value then the curves are null. Can anyone help me understand why this happens and how can I write tests without them breaking if the default value of the Movement class changes?

The class I am testing implements the following abstract class Movement:

 public abstract class Movement : ScriptableObject, IMovement
 {
     [SerializeField]
     [Tooltip("The speed at which the object moves.")]
     protected float _speed = 50f;

     public abstract void Move(Rigidbody2D rigidbody);
 }

The implementation I am currently testing is this:

 [CreateAssetMenu(menuName = "Movement/Curved Movement")]
 public class CurvedMovement : Movement
 {
     [SerializeField]
     [Tooltip("The curve that the unit will follow along the x-axis when moving.")]
     protected AnimationCurve _xCurve;

     [SerializeField]
     [Tooltip("The curve that the unit will follow along the y-axis when moving.")]
     protected AnimationCurve _yCurve;
 
     public override void Move(Rigidbody2D rigidbody)
     {
         var movementVector = new Vector2(_xCurve.Evaluate(Time.time) * _speed, _yCurve.Evaluate(Time.time) * _speed);
         rigidbody.MovePosition(rigidbody.position + movementVector * Time.fixedDeltaTime);
     }
 }

My test is here:

 [UnityTest]
         public IEnumerator CurvedMovementWithSpeedGreaterThanZeroAndNoCurveDoesNotMoveTheRigidBody()
         {
             var speed = 50f;
             var curvedMovement = ScriptableObject.CreateInstance<CurvedMovement>();
             var serializedObject = new SerializedObject(curvedMovement);
             serializedObject.FindProperty("_speed").floatValue = speed;
             serializedObject.FindProperty("_xCurve").animationCurveValue = new AnimationCurve();
             serializedObject.FindProperty("_yCurve").animationCurveValue = new AnimationCurve();
             serializedObject.ApplyModifiedProperties();
 
             var gameObject = new GameObject();
             var rigidbody2D = gameObject.AddComponent<Rigidbody2D>();
             rigidbody2D.gravityScale = 0;
             var initialPosition = rigidbody2D.position;
             curvedMovement.Move(rigidbody2D);
 
             yield return new WaitForFixedUpdate();
 
             Assert.AreEqual(initialPosition.x, rigidbody2D.position.x);
             Assert.AreEqual(initialPosition.y, rigidbody2D.position.y);
 
             GameObject.Destroy(gameObject);
         }