Why is my character's horizontal velocity lost when jumping?

Question

GOAL
I'm relatively new to Unity and I want my character to be able to run and jump at the same time causing the character to go up diagonally.

PROBLEM
However after making some adjustments to give the character some acceleration, the jump seems to clear all existing velocity, meaning the character goes up and then to the side instead of both at the same time:

(I apologise if its a bit hard to see)

CODE
This is my character movement script:

    Rigidbody2D rb;
    BoxCollider2D bc;   

    [Header("Run")]
    float xInput = 0f;
    public float maxRunSpeed;
    public float acceleration;
    [Space]

    [Header("Jump")]
    public float jumpHeight;
    public float lowJumpHeight;
    public float fallSpeed;
    public float airControl;

    [Space]
    public LayerMask groundLayer;
    public bool onGround;

    [Space]
    public Vector2 bottomOffset;
    public Vector2 boxSize;
    public float coyoteTime;

    void Start() {
        // Gets a reference to the components attatched to the player
        rb = GetComponent<Rigidbody2D>();
        bc = GetComponent<BoxCollider2D>();
    }

    void Update() {
        Jump();

        // Takes input for running and returns a value from 1 (right) to -1 (left)
        xInput =  Math.Sign(Input.GetAxisRaw("Horizontal"));
    }

    // Applies a velocity scaled by runSpeed to the player depending on the direction of the input
    // Increaces the velocity by accerleration until the max velocity is reached
    void FixedUpdate() {
        rb.velocity = Math.Abs(rb.velocity.x) < Math.Abs(xInput) * maxRunSpeed ? rb.velocity + new Vector2(acceleration * xInput, rb.velocity.y) * Time.deltaTime : new Vector2(xInput * maxRunSpeed, rb.velocity.y);   
    }

    void Jump() {
        // Checks whether the player is on the ground and if it is, replenishes coyote time, but if not, it starts to tick it down
        coyoteTime = onGround ?  0.1f :  coyoteTime - Time.deltaTime;
        // Draws a box to check whether the player is touching objects on the ground layer
        onGround = Physics2D.OverlapBox((Vector2)transform.position + bottomOffset, boxSize, 0f, groundLayer);

        // Adds an upwards velocity to player when there is still valid coyote time and the jump button is pressed
        if (Input.GetButtonDown("Jump") && coyoteTime > 0) {
            rb.velocity = Vector2.up * jumpHeight;
        }
        
        // Increases gravity of player when falling down or when the jump button is let go mid-jump
        if (rb.velocity.y < 0 ) {
            rb.velocity += Vector2.up * Physics2D.gravity.y * (fallSpeed - 1) * Time.deltaTime;
        } else if (rb.velocity.y > 0 && !Input.GetButton("Jump")) {
            rb.velocity += Vector2.up * Physics2D.gravity.y * (lowJumpHeight - 1) * Time.deltaTime;
        }
    }

Sorry for there being a lot of unecessary code, it's just im not sure what's causing the issue so i don't want to remove anything. Hopefully my comments make some sense?

Answer 1

While in all other cases you keep your velocity values and only slightly modify them you are hard overwriting the absolute velocity in

rb.velocity = Vector2.up * jumpHeight;

erasing any velocity on X.

You could simply keep whatever you have on the other axis and only overwrite Y like

var velocity = rb.velocity;
velocity.y = jumpHeight;
rb.velocity = velocity;

or

rb.velocity = new Vector2(rb.velocity.x, jumpHeight);

basically the same way you do it also in FixedUpdate for the horizontal direction.