QtConcurred spawned threads have same ID as main thread when waitForFinished() is used

Creating new threads with QtConcurrent will create new threads with ID different from the main thread, unless waitForFinished() is called.

Here is a minimal test, where I spawn a QtConcurrent on button click. The run method updates a counter on a spinbox.

#include "MainWindow.h"
#include "ui_MainWindow.h"
#include <QDebug>
#include <QThread>
#include <qtconcurrentrun.h>

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
  , ui_(new Ui::MainWindow)
  , cnt_(0)
{
  ui_->setupUi(this);
  connect(this, &MainWindow::setSpinBoxValue, ui_->spinBox, &QSpinBox::setValue);

  qDebug() << "MainWindow() thread ID: " << QThread::currentThreadId();
}

MainWindow::~MainWindow()
{
  qDebug() << "~MainWindow() thread ID: " << QThread::currentThreadId();

  delete ui_;
}

void MainWindow::on_pushButton_clicked()
{
  qDebug() << "on_pushButton_clicked() thread ID: " << QThread::currentThreadId();

  QFuture<void> th = QtConcurrent::run(this, &MainWindow::updateValue);

  // TEST MADE ENABLING/DISABLING THE FOLLOWING LINE
  th.waitForFinished();
}

void MainWindow::updateValue()
{
  qDebug() << "updateValue() thread ID: " << QThread::currentThreadId();

  emit setSpinBoxValue(++cnt_);
}

If I remove/comment th.waitForFinished() line the debug output shows different IDs for the click thread (main thread) and the concurrent thread, as expected:

14:38:17: Debugging starts
MainWindow() thread ID: 0x1ad0
on_pushButton_clicked() thread ID: 0x1ad0
updateValue() thread ID: 0xc40
on_pushButton_clicked() thread ID: 0x1ad0
updateValue() thread ID: 0xc40
on_pushButton_clicked() thread ID: 0x1ad0
updateValue() thread ID: 0xc40
~MainWindow() thread ID: 0x1ad0
14:38:27: Debugging has finished

If I wait for thread termination (th.waitForFinished() called), the concurrent runs in a new thread the first time, then runs on the main GUI thread on all successive callss:

14:40:23: Debugging starts
MainWindow() thread ID: 0x764
on_pushButton_clicked() thread ID: 0x764
updateValue() thread ID: 0x83c
on_pushButton_clicked() thread ID: 0x764
updateValue() thread ID: 0x764
on_pushButton_clicked() thread ID: 0x764
updateValue() thread ID: 0x764
~MainWindow() thread ID: 0x764
14:40:35: Debugging has finished

Why is this happening?

The behavior is correct again if I force QtConcurrent::run() to run in a new  QThreadPool:

void MainWindow::on_pushButton_clicked()
{
  qDebug() << "on_pushButton_clicked() thread ID: " << QThread::currentThreadId();

  QThreadPool pool;

  QFuture<void> th = QtConcurrent::run(&pool, this, &MainWindow::updateValue);

  th.waitForFinished();
}

Output:

19:14:28: Debugging starts
MainWindow() thread ID: 0x1b64
on_pushButton_clicked() thread ID: 0x1b64
updateValue() thread ID: 0x1084
on_pushButton_clicked() thread ID: 0x1b64
updateValue() thread ID: 0x1768
on_pushButton_clicked() thread ID: 0x1b64
updateValue() thread ID: 0x6c
on_pushButton_clicked() thread ID: 0x1b64
updateValue() thread ID: 0x7c
~MainWindow() thread ID: 0x1b64
09:14:34: Debugging has finished