Google analytics 4 events sometimes not arriving

Question

I am tracking purchases on a site in google analytics by sending a custom event from JavaScript to Google Tag Manager on the "successful purchase" page. Most of the time this works perfectly, but in some cases it seems the event just doesn't arrive to Google Analytics.

Initially I thought that maybe visiting the success page couldn't be relied on, but then I added an additional call after triggering the event that logs the sending of the event to my database. To my surprise, the events so far always get logged to my database, but they still sometimes don't show up in analytics. This is the code that does this:

                    const event = {
                        'event': 'purchase',
                        'ecommerce': {
                            'transaction_id': orderData.id,
                            'value': orderData.price,
                            'currency': 'EUR',
                            'coupon': orderData.CouponCode,
                            "items": orderData.services.map(elem => ({
                                'item_id': elem.id,
                                'item_name': elem.name,
                                'price': elem.price,
                                'item_type': elem.type,
                                'quantity': 1,
                            })),
                        }
                    };
                    
                    // Send GA4 purchase event
                    dataLayer.push(event);
                    
                    // Log to my db
                    fetch("/ajax/trackAnalytics", {
                        method: "POST",
                        headers: {
                            "Content-Type": "application/json",
                        },
                        body: JSON.stringify({
                            event,
                            cleaning_id: orderData.id
                        })
                    })

Let's take the 2nd of December as an example. According to google analytics these were the incoming purchases:

But in my database I received the following logs (I redacted the "items" field because it contained customer information but it shouldn't matter):

{
   "event":"purchase",
   "ecommerce":{
      "transaction_id":6520,
      "value":73.89,
      "currency":"EUR",
      "coupon":null
   },
   "timestamp":"2022-12-02T15:10:47+00:00"
}

{
   "event":"purchase",
   "ecommerce":{
      "transaction_id":6519,
      "value":67.99,
      "currency":"EUR",
      "coupon":null
   },
   "timestamp":"2022-12-02T15:57:44+00:00"
}

{
   "event":"purchase",
   "ecommerce":{
      "transaction_id":6487,
      "value":197.05,
      "currency":"EUR",
      "coupon":null
   },
   "timestamp":"2022-12-02T19:17:54+00:00"
}

As you can see, everything matches up except the transaction with ID 6520.

I tried creating orders that contained the exact elements 6520 did but I wasn't able to reproduce the issue that way. I also tried doing the same with a tracker blocker enabled on my browser but still the data came through.

The tag manager setup is the following:

Purchase trigger:

Purchase tag:

Answer 1

As Darrellwan suggested, adding the line window.dataLayer = window.dataLayer || []; before dataLayer.push seems to have solved the problem, but it's not 100% clear.