DispatchGroup doesn't work as expected

I have made a DispatchGroup and run 2 async tasks. One is on main and the other one is on global().

As long as I understand, DispatchGroup.notify's block should invoked once all tasks are done but it doesn't work as I thought.

class Que {
    let group = DispatchGroup()

    init() {
        group.notify(queue: .main) {
        print("group done")
        }
    }

    func run() {
        doC()
        doD()
    }

    fileprivate func doC() {
        group.enter()
        DispatchQueue.main.async(group: group) {
            var rst = 0
            for idx in 0 ..< 500 {
                rst += idx
            }
            print("work item c is done")
            self.group.leave()
        }
    }

    fileprivate func doD() {
        group.enter()
        DispatchQueue.global().async(group: group) {
            var rst = 0
            for idx in 0 ..< 50 {
                rst += idx
            }
            print("work item d is done")
            self.group.leave()
        }
    }
}

The result is

work item d is done
group done
work item c is done

I want to know why it is not

work item d is done
work item c is done
group done

If I made C and D task run on global() queue, it worked.