Cannot jump while running downhill,Character can't jump while moving down

I have a good portion of Brackeys fps character movement script implemented in a third person game I'm making. My character cannot jump while running downhill. Namely, here's what it looks like:

  if (isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
             animator.SetBool("IsJumping", false);
 
         }

The problem is that when I run downhill, the velocity on the y axis becomes much less than -2. I tried to grab the moveInput on the z axis:

 //Storing the input with the new input system 
 moveInput = new Vector3(ctx.ReadValue<Vector2>().x, moveInput.y, ctx.ReadValue<Vector2>().y);
 
 //I make a variable storing the z-axis input
 float zInput = moveInput.z;
 
 //I try to use both the y velocity and the moveInput on the z-axis to set 
 //isGrounded to be true
 if (velocity.y > 0 && zInput == 1)
 {
      Debug.Log("Y velocity is " + velocity.y);
      isGrounded = true;
 }

That doesn't work. Here's the rest of the movement script. Help a man out!

 using System;
 using System.Collections;
 using UnityEngine;
 using UnityEngine.InputSystem;
 public class PlayerMovement : MonoBehaviour
 {
     private Camera _mainCamera;
     private PlayerControls _controls;
     private CharacterController _controller;
     public Animator animator;
     public Transform groundCheck;
     public float groundDistance = 0.4f;
     public LayerMask groundMask;
 
     [Space]
     
     private float _groundedTimer;
     public float gravity = -9.81f;
     public float jumpHeight = 3f;
     public float turnSpeed = 12f;
     public bool isGrounded;
     public Vector2 mouseInput;
     public Vector3 animatorInput;
     public Vector3 moveInput;
     public Vector3 velocity;
     public float speed = 3f;
     
 
     void Awake()
     {
         //Locks the cursor to the middle of the screen
         Cursor.lockState = CursorLockMode.Locked;
 
         //Store the main camera in a variable for later use
         _mainCamera = Camera.main;
         
         //Store the player controls in a variable
         _controls = new PlayerControls();
 
         //movement keys used. This is an event
         _controls.KeyBoard.WASDKeys.performed += ctx =>
         {
             moveInput = new Vector3(ctx.ReadValue<Vector2>().x, moveInput.y, ctx.ReadValue<Vector2>().y);
         };
         
         //Movement keys no longer used. This is an event
         _controls.KeyBoard.WASDKeys.canceled += ctx =>
         {
             Debug.Log("WASD key unpressed");
             moveInput = new Vector3(ctx.ReadValue<Vector2>().x, moveInput.y, ctx.ReadValue<Vector2>().y);
         };
 
  
         //Gets the input for jumping. This is an event
         _controls.KeyBoard.SpaceBar.performed += ctx =>
         {
             Debug.Log("Space bar pressed");
             if (isGrounded && velocity.y < 0)
             {
                 animator.SetBool("IsJumping", true);
                 velocity.y = (float) Math.Sqrt(jumpHeight * -2f * gravity);
             }
         };
         //Spacebar released. This is an event
         _controls.KeyBoard.SpaceBar.canceled += ctx =>
         {
             Debug.Log("Space bar unpressed");
         };
 
         //Mouse horizontal and vertical input. This is an event
         _controls.KeyBoard.MouseDelta.performed += ctx =>
         {
             //Store mouse input into a variable for later use
             mouseInput = ctx.ReadValue<Vector2>();
         };
             
         //Store the controller in a variable
         _controller = GetComponent<CharacterController>();
         
         //Store the animator in a variable
         animator = GetComponent<Animator>();
         
         
     }
 
     //Invokes this every frame
     void Update()
     {
         float zInput = moveInput.z;
 
         //Velocity at the Y-Axis
         velocity.y += gravity * Time.deltaTime;
         
         //Boolean value for checking if the player is grounded
         bool groundedPlayer = _controller.isGrounded;
         
    
         if (groundedPlayer)
         {
             _groundedTimer = 0.2f;
         }
 
         if (_groundedTimer > 0)
         {
             _groundedTimer -= Time.deltaTime;
         }
 
         //This checks if the player is on the ground
         isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
         
     
         //If the player is grounded and the falling speed is a lot, reset the velocity at the y-axis to keep player grounded
         if (isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
             animator.SetBool("IsJumping", false);
 
         }
 
         if (velocity.y > 0 && zInput <= 1)
         {
             Debug.Log("Y velocity is " + velocity.y);
             isGrounded = true;
         }
         
     }
 
     private void LateUpdate()
     {
         Move();
         Look();
     }
 
     /// <summary>
     /// Moves the character and plays running animations according to the input
     /// </summary>
     void Move()
     {
  
         Vector3 moveVec = transform.right * moveInput.x + transform.forward * moveInput.z;
         
         _controller.Move((moveVec) * speed * Time.deltaTime);
         _controller.Move(velocity * Time.deltaTime);
     
         
         animatorInput.x = moveInput.x;
         animatorInput.z = moveInput.z;
         
         
         //SetFloat takes these parameters: name, value, dampTime, deltaTime. 
         //For keyboard input, ALWAYS add dampTime and deltaTime to the animator float values
         //in your 2D blendtrees. 
         //Do the same for joystick controls for now, unless proven otherwise.
         animator.SetFloat("InputX", animatorInput.x, 1f, Time.deltaTime * 10f);
         animator.SetFloat("InputY", animatorInput.z, 1f, Time.deltaTime * 10f);
 
     }
     
 
     /// <summary>
     /// Makes the character face the direction the camera is looking
     /// </summary>
     void Look()
     {
        //Plug into the y axis for the 
         float yawCamera = _mainCamera.transform.rotation.eulerAngles.y;
 
         transform.rotation = Quaternion.Slerp(transform.rotation,Quaternion.Euler(0,yawCamera,0),turnSpeed * Time.deltaTime);
 
         
     }
     
     //Enables and disables the controls. NEEDED.
     private void OnEnable()
     {
         _controls.Enable();
     }
        private void OnDisable()
     {
         _controls.Disable();
     }
 
   
 }