Rigidbody 2d add force horizontally affecting vertical velocity

When my character is stationary and jumps it's all ok. The vertical velocity decreases up to the peak and then increases on the way down. But when I make him move and jump the vertical velocity on the way down is linear.

This is how Im adding the force when the character is jumping

 float force = 500;
 rb.AddForce(new Vector2(1, 0) * force);

I thought maybe the addForce 0 is constantly resetting the vertical velocity so tried small values instead of 0 and I have tried using transform.right instead of creating a new Vector each time.

Im not sure what is causing it and would like some help please,

Thank you