Google Analytics "(not set)" Landing Pages

I was recently asked to investigate a site where landing page (not set) was the top page. Taking a look at a few random sites I have access to, I could see that (not set) landing page always exists, and is often among the top 3 landing pages.

The (not set) landing page happens when an event other than a page_view initiates a session.

There's no shortage of discussion on this phenomenon online and the conventional response is that it's due to resumed sessions with an event such as a scroll or other event firing before a page_view.

This would imply that all sessions landing on the (not set) page would be existing users. This was not the case on the screen shot above, but looking at the same drill down (Reports > Engagement > Landing Pages) on other GA4 properties confirmed this. For example, see that there are no new users here:

Back on the site with (not set) landing pages on both existing and new users (very top screen shot). To find the culprit event that was initiating the session before a page_view, I created a User Explorer report with a segment where dimension LandingPage exactly matches (not set). With the resulting report you can click on any user and review their event history. In each case I saw that the session_start event was occurring at the same time as a custom event which happened before page_view in the user's event stream.

This was the same for all users in the segment. I've disguised the name of the culprit event here but it was a custom event configured to fire before page_view that was causing this to happen.