Why does GA4 show “(not set)” as the landing page when the session has page_view events later?

Usually it means GA4 didn't get a usable `page_view` at the start of that session. Landing page is based on the session's first page view, not a page view that happens later, so later page views can still show up in Pages and screens while the landing page stays `(not set)`. Check for a custom event firing first, `send_page_view: false` without a prompt manual page view, consent or tag-trigger ordering, or a new session starting after timeout when an event fires first. In DebugView or BigQuery, compare the event order for one affected `ga_session_id`; the first relevant hit should include a normal `page_view` with `page_location`. A tiny amount can be edge cases, but a big chunk usually means the tag timing needs a look.