GA not recording "Sessions with Add to Basket" – Magento

Based on the comment from @lossleader to my question, I was able to identify and fix the issue. I'll answer it here in case anyone else finds it helpful.

Basically as @lossleader said in his comment the main thing is:

The custom event should also have contained the e-commerce data for the use dataLayer tag.

So triggering the addToCart tag with a simple click event is not enough as it does not contain the e-commerce data, I needed to fix how the enhance ecommerce addToCart event was triggered on Magento.

I discovered that Magento Commerce changed/fixed how they end up calling the function which trigger this event in recent upgrades, i.e. commit MAGETWO-69210 & commit MAGETWO-87437

Our Magento did receive these changes as the list.phtml template and the catalog-add-to-cart.js file were being overridden in it's theme.

Updating the theme files as the below diff shows resolved the issue in our case:

diff --git a/app/code/Namespace/CategoryPages/view/frontend/templates/product/list.phtml b/app/code/Namespace/CategoryPages/view/frontend/templates/product/list.phtml
index 6771e863..2ab8905c 100644
--- a/app/design/frontend/Namespace/theme/Magento_Catalog/templates/product/list.phtml
+++ b/app/design/frontend/Namespace/theme/Magento_Catalog/templates/product/list.phtml
@@ -91,7 +91,7 @@ $_helper = $this->helper('MagentoCatalogHelperOutput');
                                     <div class="actions-primary"<?php echo strpos($pos, $viewMode . '-primary') ? $position : ''; ?>>
                                         <?php if ($_product->isSaleable()): ?>
                                             <?php $postParams = $block->getAddToCartPostParams($_product); ?>
-                                            <form data-role="tocart-form" action="<?php /* @escapeNotVerified */ echo $postParams['action']; ?>" method="post">
+                                            <form data-role="tocart-form" data-product-sku="<?= $block->escapeHtml($_product->getSku()) ?>" action="<?= /* @NoEscape */ $postParams['action'] ?>" method="post">
                                                 <input type="hidden" name="product" value="<?php /* @escapeNotVerified */ echo $postParams['data']['product']; ?>">
                                                 <input type="hidden" name="<?php /* @escapeNotVerified */ echo Action::PARAM_NAME_URL_ENCODED; ?>" value="<?php /* @escapeNotVerified */ echo $postParams['data'][Action::PARAM_NAME_URL_ENCODED]; ?>">
                                                 <?php echo $block->getBlockHtml('formkey')?>
diff --git a/app/design/frontend/Namespace/theme/Magento_Catalog/web/js/catalog-add-to-cart.js b/app/design/frontend/Namespace/theme/Magento_Catalog/web/js/catalog-add-to-cart.js
index fae6f0fa..34978ec4 100644
--- a/app/design/frontend/Namespace/theme/Magento_Catalog/web/js/catalog-add-to-cart.js
+++ b/app/design/frontend/Namespace/theme/Magento_Catalog/web/js/catalog-add-to-cart.js
@@ -6,8 +6,10 @@
 define([
     'jquery',
     'mage/translate',
+    'underscore',
+    'Magento_Catalog/js/product/view/product-ids-resolver',
     'jquery/ui'
-], function ($, $t) {
+], function ($, $t, _, idsResolver) {
     'use strict';

     $.widget('mage.catalogAddToCart', {
@@ -75,7 +77,9 @@ define([
          * @param {String} form
          */
         ajaxSubmit: function (form) {
-            var self = this;
+            var self = this,
+                productIds = idsResolver(form),
+                formData = new FormData(form[0]);

             $(self.options.minicartSelector).trigger('contentLoading');
             self.disableAddToCartButton(form);
@@ -97,6 +101,13 @@ define([
                 success: function (res) {
                     var eventData, parameters;

+                    $(document).trigger('ajax:addToCart', {
+                        'sku': form.data().productSku,
+                        'productIds': productIds,
+                        'form': form,
+                        'response': res
+                    });
+
                     if (self.isLoaderEnabled()) {
                         $('body').trigger(self.options.processStop);
                     }