A step vanishes when you step off it?

Here is something that works. I have a player object with a rigidbody and collider. I have steps with rigidbodies that use no gravity and are kinematic and a collider.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 
 // Simple player mover that just moves it forward so i can move down steps
 public class PlayerController : MonoBehaviour
 {
     Rigidbody rb;
 
 
     private void Awake()
     {
         rb = GetComponent<Rigidbody>();
     }
 
 
     private void Update()
     {
         if (Input.GetKey(KeyCode.RightArrow))
         {
             rb.MovePosition(transform.position + transform.forward * Time.deltaTime * 1f);
         }
     }
 }

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 
 //This is placed on your steps
 public class StepController : MonoBehaviour
 {
     bool isPlayerOn;
 
 
     private void OnCollisionEnter(Collision collision)
     {
         // Is it a player - using markup (this makes sure other things doesn't hit the step and turn it off
         PlayerController player = collision.gameObject.GetComponent<PlayerController>();
         if (player)
         {
             isPlayerOn = true;
         }
     }
 
 
     private void OnCollisionExit(Collision collision)
     {
         if (isPlayerOn)
         {
             gameObject.SetActive(false);
             isPlayerOn = false;
         }
     }
 }

Worked fine for me, hopefully it gives you some idea of doing it.