QtConcurrent and recursion

Sanych.by

Good day. I tried execute some long operation which use recursion with QtConcurrent::run. But signal finished() of QFutureWatcher not emitted for all calls. Here code:
MainWIndow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QThreadPool>

namespace Ui {
class MainWindow;
}

class WatcherHandler: public QObject
{
    Q_OBJECT
public slots:
    void    watcherFinished();
};

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();
    QString recurse(int level, int count = 0);
private:
    QThreadPool *pool;
    int MaxLevel;
    Ui::MainWindow *ui;
    WatcherHandler *h;
};
#endif // MAINWINDOW_H

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtConcurrent>
#include <QDebug>
#include <QThread>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    pool = nullptr;
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
    if(pool)
        delete pool;
}

void MainWindow::on_pushButton_clicked()
{
    QThread *thrd = new QThread;
    h = new WatcherHandler;
    h->moveToThread(thrd);
    connect(qApp, &QApplication::aboutToQuit,
            thrd, &QThread::deleteLater);
    thrd->start();
    pool = new QThreadPool;
    MaxLevel = 3;
    //QtConcurrent::run(pool, this, &MainWindow::recurse,0,0);
    recurse(0);
}

QString MainWindow::recurse(int level, int count)
{
    qDebug() << "Thread : " << QThread::currentThread() << " : " << thread();
    qDebug() << "Start recurse level " << level << count;
    unsigned long worktime = 1000/(level+1)-level;
    QThread::currentThread()->msleep(worktime);
    if(level < MaxLevel)
        for(int i = 0; i <= level; i++)
        {
            QFutureWatcher<QString> *watcher = new QFutureWatcher<QString>;
            connect(watcher, &QFutureWatcher<QString>::finished,
                    h, &WatcherHandler::watcherFinished,
                    Qt::QueuedConnection);
            QFuture<QString> f =
                    QtConcurrent::run(pool, this, &MainWindow::recurse,
                                                   level+1, i);
            watcher->setFuture(f);

        }
    QThread::currentThread()->msleep(worktime);
    qDebug() << "Finished recurce level" << level << count;
    return QString("level %1 count %2").arg(level).arg(count);
}

void WatcherHandler::watcherFinished()
{
    QFutureWatcher<QString> *watcher = static_cast<QFutureWatcher<QString> *>
            (QObject::sender());
    qDebug() << "Thread : " << QThread::currentThread();
    qDebug() << "SIGNAL FINISHED EMITED FOR" << watcher->result();
}

And debug log:
Thread : QThread(0x3da9c0) : QThread(0x3da9c0)
Start recurse level 0 0
Thread : QThread(0x42b430, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 1 0
Thread : QThread(0x463950, name = "Thread (pooled)") : QThread(0x3da9c0)
Thread : QThread(0x463b80, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 2 0
Start recurse level 2 1
Thread : QThread(0x463db0, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 3 0
Finished recurce level 1 0
Thread : QThread(0x42b430, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 3 0
Finished recurce level 0 0
Thread : QThread(0x466890)
SIGNAL FINISHED EMITED FOR "level 1 count 0"
Finished recurce level 2 0
Finished recurce level 2 1
Thread : QThread(0x463950, name = "Thread (pooled)") : QThread(0x3da9c0)
Thread : QThread(0x463b80, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 3 1
Start recurse level 3 1
Finished recurce level 3 0
Thread : QThread(0x463db0, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 3 2
Finished recurce level 3 0
Thread : QThread(0x42b430, name = "Thread (pooled)") : QThread(0x3da9c0)
Start recurse level 3 2
Finished recurce level 3 1
Finished recurce level 3 1
Finished recurce level 3 2
Finished recurce level 3 2

Where is my fall?
P.S. I add qDebug for connect call, always return true. Seems like only first recursion deep emitted. But in real example i got also signals for second level (maybe it depends on time of operation). And for last 3-5 calls of second level (all calls second level was about 180-190) I never got finished signal.

Sanych.by

Ok. I I made it work after making QThread object a member and moving each QFutureWatcher object to it. I guess something wrong was in creating watcher in thread of QtConcurrent::run and when one call of recurse with QtConcurrent::run is finished, all watchers created in it, even dynamically and without a parent, were lost, or their connections were lost.
I don't understand why it is so but it works. If someone can explain me this situation and why it works this way, i would be very thankful for explainations, so i would use CONNECT mechanism right in future.