Missing orders in GA from Firefox in private mode

[blazejstarosta]

My website is using this library to implement Measurement Protocol for three actions: add to cart, remove from cart and purchase (sending the order to GA). The rest of Google Analytics (like page views, impressions and so on) are integrated with Google Tag Manager. Everything is working fine except for sending orders. We noticed we are missing some orders in our default view in GA.

In our GA we have two views - the default one and UID. We are not actually using UID, we thought about doing it earlier but ended up ditching the idea. The view remained, though. However, we noticed that the UID view contains all orders, including the ones missing from the standard view.

We started doing more research and, in the end, we found out what's going on. All missing orders from the default view come from Firefox in private mode. Also, incognito mode in the new Edge (the one based on Chromium) also seems to cause the same issue. Whenever we place a test order from either Firefox or Edge in private mode the order will not be visible in the standard view, it will only show up in the UID view. It only affects these two browsers. The orders from the private mode in Safari or Chrome appear properly in both views.

Does anyone have any idea what might be causing it and how it is even possible?