The sticky payment section in the Storefront Theme can be a little buggy. Sometimes it’s best to remove it.
Here is how to make WooCommerce payment section non sticky. Just drop the code into your child themes functions.php

// Removes the sticky scrolling payment section on WooCommerce checkout
function wpsnippets_remove_sticky_wc_payment() {
    wp_deregister_script( 'storefront-sticky-payment');
}
add_action( 'wp_enqueue_scripts', 'wpsnippets_remove_sticky_wc_payment' , 90 );