Where is this text coming from at the end of my checkout URL?

Question

I'm adding a "buy now" button to my site. I'm using the code from here.

When I click the button, my site tries to go to example.com/checkout/#93=4, which then results in it going to example.com/checkout/noroute. Where is the #93=4 part coming from? I've tried to step through the Magento code in a debugger, and as far as I can tell, the extension is trying to go to example.com/checkout/, and I can't figure out what's adding the #93=4. But I'll be honest; I don't understand Magento's routing.

Here are the parts of the code that I think are relevant:

buynow.js

define([
    'jquery'
], function ($) {
    "use strict";
    return function (config, element) {
        $(element).click(function () {
            var form = $(config.form);
            var baseUrl = form.attr('action'),
            buyNowUrl = baseUrl.replace('checkout/cart/add', 'buynow/cart/add');
            form.attr('action', buyNowUrl);
            form.trigger('submit');
            form.attr('action', baseUrl);
            return false;
        });
    }
});

Add.php

I've used the debugger to verify that the line $baseUrl = $this->_objectManager->get('\Magento\Store\Model\StoreManagerInterface')->getStore()->getBaseUrl(); towards the bottom of the code does return example.com/checkout/ (no #93=4)

class Add extends \Magento\Checkout\Controller\Cart\Add
{
    /**
     * Add product to shopping cart action
     *
     * @return \Magento\Framework\Controller\Result\Redirect
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     */
    public function execute()
    {
        if (!$this->_formKeyValidator->validate($this->getRequest())) {
            return $this->resultRedirectFactory->create()->setPath('*/*/');
        }

        $params = $this->getRequest()->getParams();
        try {
            if (isset($params['qty'])) {
                $filter = new \Zend_Filter_LocalizedToNormalized(
                    ['locale' => $this->_objectManager->get('Magento\Framework\Locale\ResolverInterface')->getLocale()]
                );
                $params['qty'] = $filter->filter($params['qty']);
            }

            $product = $this->_initProduct();
            $related = $this->getRequest()->getParam('related_product');

            if (!$product) {
                return $this->goBack();
            }

            $cartProducts = $this->_objectManager->create('Test\BuyNowButton2\Helper\Data')
                             ->getConfig('buynow/general/keep_cart_products');
            if (!$cartProducts) {
                $this->cart->truncate(); //remove all products from cart
            } 

            $this->cart->addProduct($product, $params);
            if (!empty($related)) {
                $this->cart->addProductsByIds(explode(',', $related));
            }

            $this->cart->save();
            $this->_eventManager->dispatch(
                'checkout_cart_add_product_complete',
                ['product' => $product, 'request' => $this->getRequest(), 'response' => $this->getResponse()]
            );

            if (!$this->_checkoutSession->getNoCartRedirect(true)) {
                $baseUrl = $this->_objectManager->get('\Magento\Store\Model\StoreManagerInterface')
                            ->getStore()->getBaseUrl();
                return $this->goBack($baseUrl.'checkout/', $product);
            }
        } catch (\Exception $e) {
            // ...
        }
    }
}