Strange bug in a Space Invaders type game

Hi,i am new to unity and i am trying to create a game like Space Invaders, but i have a bug in the movement script of the invaders. in the Spawn script that is attached to an empty gameobject(scene controller), i create 40 invaders by instantiate the movement serializedfield and i fix their position, then i memorize all the invaders in a list; Once done with that i pass this gameobject in the movement script (that is attached to each of invaders) as serializedfield so i can retieve the list with all the invaders;here first i create a function to controll the object with max pos and with less pos, so i find the invader of the right border and of the left border; in the update method i move the invader since the invader of right border has an position <18, when is>18 i change the speed to negative, and the same for the other side. the problem is that when i run the game only the row with the invader of right border and left border acts in a strange way, when the right border arrives at 18 and changes his speed to negative , all the invaders of the row change their position a little to left and same happens for the other side;

as you can see only the row with the two invaders that i use for changing velocity after a few time shifts without a reason,this movement script;

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Linq;
 
 public class InvaderMovement : MonoBehaviour
 {
     [SerializeField]private Spawner a;
 
     public float MinX = -18;
     public float MaxX = 18;
     public float speed=90;
     private int b = 1;
     private int indexMAX;
     private int indexMIN;
     private bool Dir = false;
 
     void Start()
     {
         speed = -3;
         confrontall();
         Debug.Log(indexMAX);
         Debug.Log(indexMIN);
         Debug.Log(a.AllInvaders[indexMAX].transform.position.x);
 
     }
 
     void Update()
     {
         transform.Translate(new Vector3(speed, 0, 0));
 
         if (a.AllInvaders[indexMAX].transform.position.x >= 18)
         {
             speed = -speed;
             Debug.Log(speed);
             Debug.Log("cio");
             
             Dir = true;
         }
 
         if (a.AllInvaders[indexMIN].transform.position.x <= -18 )
         {
             Debug.Log(speed);
             speed = Mathf.Abs(speed);
             Dir = false;
         }
     }
 
     public void confrontall()
     {
             float max = -1000;
     float min = 1000;
 
         for(int j=0; j<a.AllInvaders.Count;j++)
         {
             if (a.AllInvaders[j] != null)
             {
                 if (a.AllInvaders[j].transform.position.x > max)
                     max = a.AllInvaders[j].transform.position.x;
                 if (a.AllInvaders[j].transform.position.x < min)
                     min = a.AllInvaders[j].transform.position.x;
             }
 
         }
         for (int i=0; i<a.AllInvaders.Count;i++)
         {
             if (a.AllInvaders[i] != null)
             {
                 if (max == a.AllInvaders[i].transform.position.x)
                     indexMAX = i;
                 if (min == a.AllInvaders[i].transform.position.x)
                     indexMIN = i;
             }
 
         }
     }
 
 }
 

this is the Spawn script;

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Spawner : MonoBehaviour
 {
     [SerializeField] private InvaderMovement invader;
     private InvaderMovement obj;
     internal List <InvaderMovement> AllInvaders = new List<InvaderMovement>();
     private float offsetX = 3;
     private float offsetY = 2;
     private int k = 0;
 
     void Awake()
     {
         int max = 5;
         Vector3 OriginalPos = invader.transform.position;
         for (int i = 0; i < 4; i++)
         {
             max = 10;
              OriginalPos = invader.transform.position;
                 OriginalPos.x = OriginalPos.x - offsetX;
             for (int j = 0; j < max; j++)
             {
                 AllInvaders.Add(Instantiate(invader) as InvaderMovement);
                 AllInvaders[k].transform.position = new Vector3(OriginalPos.x + offsetX, OriginalPos.y -(i * offsetY), OriginalPos.z);
                 OriginalPos.x = AllInvaders[k].transform.position.x;
                 k++;
             }
         }
         Destroy(invader.gameObject);
     }
     
 }

here i also put the inspector of the two object. I hope someone can help me