QWebEngineCookieStore::setCookie will change the cookie domain by adding a leading dot

QList<QNetworkCookie> cookies;

QObject::connect(Browser->page()->profile()->cookieStore(), &QWebEngineCookieStore::cookieAdded, [this](const QNetworkCookie &cookie) {
    cookies.append(cookie);
    qDebug() << "Cookie Added -" << cookie.domain() << cookie.name() << endl;
});

// First visit, the cookies was saved correctly (the server sent any cookies with domain - aa.bb.cc.com).
Browser->load(QUrl("https://aa.bb.cc.com"));

// Next time to re-visit it with the saved cookies
Browser->page()->profile()->cookieStore()->deleteAllCookies();
for (auto cookie : cookies) {
    Browser->page()->profile()->cookieStore()->setCookie(cookie);
    or
    Browser->page()->profile()->cookieStore()->setCookie(cookie, QUrl("https://aa.bb.cc.com"));
    qDebug() << "[account] set>>" << cookie.domain() << cookie.name() << endl;
}
Browser->load(QUrl("https://aa.bb.cc.com"));
// the cookie domain has been changed by QWebEngineCookieStore::setCookie, it will automatically add a leading dot to the domain

Result:
// first visit
Cookie Added - "aa.bb.cc.com" "uuid"

// second visit
[account] set>> "aa.bb.cc.com" "uuid"
Cookie Added - ".aa.bb.cc.com" "uuid"