KinematicBody2D Ramp Glitching

Question

For some reason, when I go over a ramp the first time, everything works. But after a few times of going over, the ramp doesn't work and my player glitches. Here is my code:

velocity.y += delta * GRAVITY

if Input.is_action_pressed("ui_left"):
    if is_on_floor():
        velocity.x = -WALK_SPEED
    else:
        velocity.x = -WALK_SPEED / 2
elif Input.is_action_pressed("ui_right"):
    if is_on_floor():
        velocity.x = WALK_SPEED
    else:
        velocity.x = WALK_SPEED / 2
elif is_on_floor():
    velocity.x = 0

move_and_slide(velocity, Vector2(0, -1))