Full Answer

Start with what Safari leaves alone. The GA4 script loads, pageviews fire, and session data collects normally, which is why the problem hides in plain sight. What changes is cookie lifetime. Under Intelligent Tracking Prevention, the first-party _ga cookie that normally persists for two years is capped at seven days, and any visit that arrives with link decoration such as a gclid drops that ceiling to 24 hours.

That cap is where measurement breaks. When a Safari visitor returns after the window closes, GA4 has no prior identifier to match them against, so it files them as a new user. Returning customers become phantom first-timers, unique-visitor counts inflate, and any conversion that takes longer than a week to close loses its original source. For a store running paid traffic, the ad-click 24-hour cap is worse, because it severs the click from the eventual purchase entirely.

The reason client-side fixes do not help is that the limit lives in the browser, not in your tag setup. No amount of GA4 configuration extends a cookie Safari has already decided to expire. The durable answer is to move identity and event collection to a first-party server you control, where the visitor relationship is recorded server-side and sent to GA4 through the Measurement Protocol, independent of what Safari does to browser cookies. That is the difference between measuring around the restriction and pretending it isn't there.