Magento – Can you tell me from where it comes discount in Magento 2 order items

This rule doesn't work because Magento 2 doesn't save payment method to quote when you select one. And it also doesn't reload totals when selecting a payment method. And unfortunately, you have to write a custom module to solve the issue.

The new module needs only 4 files to be created:

1. app/code/Namespace/ModuleName/etc/frontend/routes.xml

   <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:App/etc/routes.xsd">
       <router id="standard">
           <route id="namespace_modulename" frontName="namespace_modulename">
               <module name="Namespace_ModuleName"/>
           </route>
       </router>
   </config>
   

   This will define a new controller for our module.

2. app/code/Namespace/ModuleName/Controller/Checkout/ApplyPaymentMethod.php

   <?php
   
   namespace Namespace\ModuleName\Controller\Checkout;
   
   class ApplyPaymentMethod extends \Magento\Framework\App\Action\Action
   {
       /**
        * @var \Magento\Framework\Controller\Result\ForwardFactory
        */
       protected $resultForwardFactory;
   
       /**
        * @var \Magento\Framework\View\LayoutFactory
        */
       protected $layoutFactory;
   
       /**
        * @var \Magento\Checkout\Model\Cart
        */
       protected $cart;
   
       /**
        * @param \Magento\Framework\App\Action\Context $context
        * @param \Magento\Framework\View\LayoutFactory $layoutFactory
        * @param \Magento\Framework\Controller\Result\ForwardFactory $resultForwardFactory
        */
       public function __construct(
           \Magento\Framework\App\Action\Context $context,
           \Magento\Framework\Controller\Result\ForwardFactory $resultForwardFactory,
           \Magento\Framework\View\LayoutFactory $layoutFactory,
           \Magento\Checkout\Model\Cart $cart
       ) {
           $this->resultForwardFactory = $resultForwardFactory;
           $this->layoutFactory = $layoutFactory;
           $this->cart = $cart;
   
           parent::__construct($context);
       }
   
       /**
        * @return \Magento\Framework\Controller\ResultInterface
        */
       public function execute()
       {
           $pMethod = $this->getRequest()->getParam('payment_method');
   
           $quote = $this->cart->getQuote();
   
           $quote->getPayment()->setMethod($pMethod['method']);
   
           $quote->setTotalsCollectedFlag(false);
           $quote->collectTotals();
           $quote->save();
       }
   }
   

   This file creates controller action to save the selected payment method to quote

3. app/code/Namespace/ModuleName/view/frontend/requirejs-config.js

   var config = {
       map: {
           '*': {
               'Magento_Checkout/js/action/select-payment-method':
                   'Namespace_ModuleName/js/action/select-payment-method'
           }
       }
   };
   

   This file allows to override Magento_Checkout/js/action/select-payment-method file

4. app/code/Namespace/ModuleName/view/frontend/web/js/action/select-payment-method.js

   define(
       [
           'Magento_Checkout/js/model/quote',
           'Magento_Checkout/js/model/full-screen-loader',
           'jquery',
           'Magento_Checkout/js/action/get-totals',
       ],
       function (quote, fullScreenLoader, jQuery, getTotalsAction) {
           'use strict';
           return function (paymentMethod) {
               quote.paymentMethod(paymentMethod);
   
               fullScreenLoader.startLoader();
   
               jQuery.ajax('/namespace_modulename/checkout/applyPaymentMethod', {
                   data: {payment_method: paymentMethod},
                   complete: function () {
                       getTotalsAction([]);
                       fullScreenLoader.stopLoader();
                   }
               });
   
           }
       }
   );
   

   Sends ajax request to save payment method and reload cart totals.

P.S. Parts of the code were taken from Payment Fee extension for Magento 2.

We can use \Magento\Quote\Model\Quote\Item $item to check the applied rules.

vendor/magento/module-sales-rule/Model/Quote/Discount.php

 $item->getAppliedRuleIds();
 $item->getDiscountAmount();

See a good example here: vendor/magento/module-sales-rule/Observer/SalesOrderAfterPlaceObserver.php

In database, we should take a look at quote_item table.