Accels Instead of Jumping?

I have this issue now for days and it is driving me crazy.. Using a very simple Charactercontroller with this snippet:

 using UnityEngine;
 using System.Collections;
 
 public class FPS : MonoBehaviour {
 
     public float speed = 20.0f;
     public float jumpSpeed = 10.0f;
     public float gravity = 25.0f;
     
     public float mouseSensitivity = 5f;
     public float upDownRange = 45f;
     
     private Vector3 moveDirection = Vector3.zero;
     private bool grounded = false;
     
     private float verticalRotation = 0f;
     
     void Update () {        
         transform.Rotate (0, Input.GetAxis ("Mouse X") * mouseSensitivity, 0);
         verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
         verticalRotation = Mathf.Clamp (verticalRotation, -upDownRange, upDownRange);
         Camera.main.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);
     }
     
     void FixedUpdate() {
         
         if (grounded) {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             
             if (Input.GetButton ("Jump")) {
                 moveDirection.y = jumpSpeed;
             }
         }
         
         moveDirection.y -= gravity * Time.deltaTime;
         
         CharacterController controller = GetComponent<CharacterController>();
         CollisionFlags flags = controller.Move(moveDirection * Time.deltaTime);
         grounded = (flags & CollisionFlags.CollidedBelow) != 0;
         
         
         if (transform.position.y < 0) {
             transform.position = new Vector3 (transform.position.x, 256, transform.position.z);
         }
         
     }
 }

But instead of a nice jump, the char only jumps when standing still - when moving around the char gets boosted into the direction it is heading and only going up when it hits a wall ?? I tried EVERYTHING millions of different approaches but the outcome is the same.

To test this just: - make a scene, add a plane - add a cube or something - add charactercontroller to the cube - attach the camera to the cube - add this script in the post to the cube

Walk and 'jump'... Is this a bug?