All follow functionality should be implemented in LateUpdate() method. See manual.

Example using your code:

void LateUpdate {
    Vector3 screenPoint = _camera.WorldToScreenPoint(transform.parent.position); // the parent is the enemy, I attached the script to the child canvas "HealthBar"
    _healthBarReact.position = screenPoint;
}

All UI follow functionality should be implemented in LateUpdate() method. See manual.

Example using your code:

void LateUpdate {
    Vector3 screenPoint = _camera.WorldToScreenPoint(transform.parent.position); // the parent is the enemy, I attached the script to the child canvas "HealthBar"
    _healthBarReact.position = screenPoint;
}