My object didn't follow a target from my script that spawn the said object?

Question

This is my object that supposed to follow my target like a homing missile:

public Transform target;
public float movementSpeed;
public float rotataSpeed;

private Rigidbody2D rb;

private void Start()
{
    rb = GetComponent<Rigidbody2D>();
}

private void FixedUpdate()
{
    Vector2 direction = (Vector2)target.position - rb.position;
    direction.Normalize();

    float rotateAmount = Vector3.Cross(direction, -transform.up).z;

    rb.angularVelocity = -rotateAmount * rotataSpeed;
    rb.velocity = -transform.up * movementSpeed;
}

which works fine as the object itself but when I put it under a script that spawns this object, it work funny like it didn't really track the target when it first launch but then it did the U-turn towards my target. I want it to track my target when it first spawn. This is my spawn script:

float secondsbetweenSpawn = Mathf.Lerp(secondsBetweenSpawnMinMax.x, secondsBetweenSpawnMinMax.y, Difficulty.GetDifficultyPercentage());
if (secondsbetweenSpawn < 1.30f)
{
    nextSpawnTime = Time.time + secondsbetweenSpawn;
    float spawnPillSize = Random.Range(SpawnSizeMinMax.x, SpawnSizeMinMax.y);
    Vector2 spawnPosPill = new Vector2(Random.Range(-screenSize.x, screenSize.x), screenSize.y);
    GameObject pillDrop = (GameObject)Instantiate(Pill, spawnPosPill, Quaternion.identity);
    pillDrop.transform.localScale = Vector2.one * spawnPillSize;
 }