Why is KinematicBody Compounding Gravity When Moving

Question

I've been trying to use KinematicBody for character movement, but It's behaving pretty unexpectedly. It's compounding the force of gravity but only when in motion. here's the entire script:

extends KinematicBody

var velocity = Vector3.ZERO
var max_acceleration = 100
var max_speed = 10
var max_ground_angle = 45
var movement_input = Vector2.ZERO
var snap_normal = Vector3.DOWN
var snap_distance = 0.2

func _process(delta):
    movement_input = Vector2(
        lerp(movement_input.x, 
        Input.get_action_strength("move_right") - 
        Input.get_action_strength("move_left"),
        0.4),
        lerp(movement_input.y,
        Input.get_action_strength("move_down") - 
        Input.get_action_strength("move_up"),
        0.4))

    movement_input.x = clamp(movement_input.x, -1, 1)
    movement_input.y = clamp(movement_input.y, -1, 1)
func _physics_process(delta):

    var gravityNormal = ProjectSettings.get_setting("physics/3d/default_gravity_vector")
    var gravityStep = (gravityNormal * ProjectSettings.get_setting("physics/3d/default_gravity")) * delta
    velocity += gravityStep

    var max_speed_change = max_acceleration * delta
    var desired_velocity = (Vector3(movement_input.x, 0, movement_input.y) * max_speed)

    velocity.x = desired_velocity.x
    velocity.z = desired_velocity.z

    velocity = move_and_slide(velocity, Vector3.UP, true, 4, deg2rad(max_ground_angle))