This is very niche, and is mainly of interest if you're using the Webtoffee Cookie consent plugin (there is a free version available too). However, if you're interested in debugging analytics problems, then this gives you some techniques.
The problem showed itself that the engagement time on our GA4 analytics was zero. As you can see from the graph above, not exactly zero, but certainly very small. I turned to webmasters.stackexchange.com and got a response from jen. It didn't give me the answer, but pointed me to an analytics debugger extension for Chrome. I could see that using that the engagment time (_et parameter) wasn't zero, but was a small number of milliseconds, less than 50.
I tried a number of things on the cookie plugin settings, such as putting the Google Tag Manger code into cookie categories, as recommend by this article (only for the free plugin though). That made the analytics script run when you accepted the cookies, but never after that. I tried disabling it, but it still didn't make engagement time get registered.
In the end though, the problem was that there was code in our site theme designed to work with GTM, but we hadn't been asked to do the corresponding changes in our GTM config. Once I removed that code the engagement time was a healthy number of seconds. I don't quite know why that caused this problem, but my theory is that the events it was triggering ran pretty quickly after the page loaded, and at that point the user engagement was recorded on those events, rather than when the user subsequently did something (scroll, navigate).
Other things to help with debugging
- Chrome application tab, cookie section, has a handy clear all cookies button to the right of the filter box.
- Make sure you enable the analytics debugger extension when trying to debug!
- Clear the site cache after making changes to the cookie plugin settings
