im making an endless runner but my capsule starts floating after i jump. any help?

im making an endless runner but whenever i jump the capsule floats above the platforms. I used character controller for the movement.

Here's the code for the character controller:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 public class herocharactercontroller : MonoBehaviour
 {
     [SerializeField] LayerMask groundLayers;
     [SerializeField] private float runspeed = 8f;
     [SerializeField] private float jumpHeight = 2f;
     private float gravity = -50f;
     private CharacterController characterController;
     private Vector3 velocity;
     private bool isGrounded;
     private float HorizontalInput;
 
     void Start()
     {
         characterController = GetComponent<CharacterController>();
     }
 
     void Update()
     {
         HorizontalInput = 1;
 
         //face forward
         transform.forward = new Vector3(HorizontalInput, 0, Mathf.Abs(HorizontalInput) - 1);
 
         //isGrounded
         isGrounded = Physics.CheckSphere(transform.position, 0.1f, groundLayers, QueryTriggerInteraction.Ignore);
         if (isGrounded && velocity.y < 0)
         {
             velocity.y = 0;
         }
         else
         {
             //add gravity
             velocity.y += gravity * Time.deltaTime;
         }
         characterController.Move(new Vector3(HorizontalInput * runspeed, 0, 0) * Time.deltaTime);
         if (isGrounded && Input.GetButtonDown("Jump"))
         {
             velocity.y += Mathf.Sqrt(jumpHeight * -2 * gravity);
         }
 
         //vertical velocity
         characterController.Move(velocity * Time.deltaTime);
     }
 }