After using transform.position, gameobject resets z-axis position

Hi everyone.

I'm trying to do something "simple": to make the gameobject move towards the player's touch (on the X-Y plane), while moving the gameobject along the Z-axis at a constant speed. The first part, I've managed. The second, not so much.

 using UnityEngine;
 
 public class Player : MonoBehaviour
 {
     //flag to check if the user has tapped
     //Set to true on tap. Reset to false on reaching destination
     private bool shouldmove = true;
     //destination point
     private Vector3 endPoint;
     private float zPosition;
 
     Ray ray;
     RaycastHit raycast;
 
     public float duration = 50f;
     public float FwSpeed = 1f;
 
     private Rigidbody _rigidbody;
 
     void Start()
     {
         zPosition = _rigidbody.position.z;
     }
     
     void Awake()
     {
         _rigidbody = GetComponent<Rigidbody>();
     }
 
     void Update()
     {
         if(Input.touchCount > 0 && (Input.GetTouch(0).phase == TouchPhase.Began || Input.GetTouch(0).phase == TouchPhase.Moved))
         {
             ray = Camera.main.ScreenPointToRay(Input.GetTouch(0).position);
 
             if(Physics.Raycast(ray,out raycast))
             {
                 endPoint = raycast.point;
                 endPoint.z = zPosition;
                 Debug.Log(endPoint);
                 shouldmove = true;
             }
         }
 
 
         if(shouldmove && !Mathf.Approximately(_rigidbody.position.magnitude, endPoint.magnitude))
         {
             _rigidbody.transform.position = Vector2.Lerp
                 (
                 _rigidbody.position,
                 endPoint ,
                 1/(duration*(Vector2.Distance(_rigidbody.position, endPoint)))
                 );
         }
 
         else if(shouldmove && Mathf.Approximately(_rigidbody.position.magnitude, endPoint.magnitude))
         {
             shouldmove = false;
             Debug.Log("I am here");
         }
         _rigidbody.velocity = new Vector3(_rigidbody.velocity.x, _rigidbody.velocity.y, FwSpeed);
         zPosition = _rigidbody.position.z;
     }
 }

For some reason, it resets the gameobject.position.z to 0 after the transform.position. Any clue to as to why and how to fix it?