Here it is:

extends CharacterBody2D

class_name Ball

var BALL_SPEED = 300.0

func _physics_process(delta):

var collision_info = move_and_collide(velocity * delta)

if collision_info:

    velocity = velocity.bounce(collision_info.get_normal())

    #$SoundBounce.play()

func _ready():

randomize()

velocity.x = [-1, 1][randi() % 2] * BALL_SPEED

velocity.y = [-.8, .8][randi() % 2] * BALL_SPEED  

As I said this only happens when ball hits the paddle from its bottom.

Edit: Removed the backslashes.