Persistent cookie being deleted as soon as Safari is closed and reopened (working fine in Chrome, Firefox)

Question

I'm using the following JavaScript code to store a cookie with a ID in the browser of the website visitor for 60 days.

While it is working perfectly in Chrome, Firefox etc. in Safari the cookie disappears after closing and reopening the browser. (happens with iPhones, Macbooks etc., even in the standard cookie-configuration)

function setCookie() {
    const date = new Date;
    date.setDate(date.getDate() + 60);
    var uid = Date.now().toString(36) + Math.random().toString(36).substring(2);
    document.cookie = "ck=" + uid + "; expires=" + date + "; path=/";
}

Does anyone have an idea, why this could be happening? Greetings!