Seems a little code alternation might do the trick.

Unlike QDirModel, QFileSystemModel uses a separate thread to populate itself so it will not cause the main thread to hang as the file system is being queried. Calls to rowCount() will return 0 until the model populates a directory.

It says it's using a separate thread. You said you tried my suggestion of making the debug calls to the functions not start till a second after the expansion and that did make them return the correct answers. If you put that together with the separate thread....

I think you should try putting a slot on the http://doc.qt.io/qt-5/qfilesystemmodel.html#directoryLoaded signal. Let's see it report on which directories it finished loading when? Compare when those signals arrive to the value of your rowCount() etc.

If that indeed explains the behaviour, you may have to rewrite your code a bit so that you use that signal in combination with the expanded one so that you can get your results reliably?

Not sure though. Can you please provide your insight .

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

    model = new QFileSystemModel;
    model->setRootPath("");

    ui->treeView->setModel(model);
    ui->treeView->setRootIndex(model->index(""));
    connect(ui->treeView,SIGNAL(expanded(QModelIndex)),this,SLOT(func(QModelIndex)));
}

void MainWindow::func(QModelIndex index)
{
      qDebug() << model->data(index1,Qt::DisplayRole);
     qDebug() << model->hasChildren(index1);
     qDebug() << model->hasIndex(0,0,index1);
     qDebug() << model->rowCount(index1);
}

Is Media a system disk or an external storage ?

I wondered about this (slow external media gets updated "slowly" under Windows). That's why I asked the OP when it happens

Does this behaviour happen on any directory node in the tree, or only certain ones (like, only volume ones)?

but he says

It happens on every directory node in the tree...

so assuming that's true it's not to do with media type.

Can this be a bug ???

Can you provide a minimum example to replicate the problem? if we can confirm the bug we cat take a look if it's solvable

Is Media a system disk or an external storage ?

your workaround suggestion with single-shot timer , works fine.

So, let's be clear. You only get (normally) get a count of 0 first time. If you make it so the first time you examine the children is a second after the expansion was performed it does produce the correct count. Is that what you are saying?

If it happens on every directory, my impression is there would be code out there which simply does not work. It's like your expand() is being called at the very start of the expansion, but it's the expanded slot and that should be called after the expand has been acted on, I assume.

Does this behaviour happen on any directory node in the tree, or only certain ones (like, only volume ones)?

It happens on every directory node in the tree...

Can this be a bug ???

We don't know yet. Does it matter? At this point I assume you're looking for a workaround.

Given that @VRonin's suggestions have not worked, at this point if it were me: I would test your code which outputs the numbers by calling it from your expand() via a single-shot QTimer with a delay of, say, a second. Let's see whether by any chance there's some OS activity going on which does not properly expand a top-level volume directory immediately and only really does it asynchronously? Does this behaviour happen on any directory node in the tree, or only certain ones (like, only volume ones)?

Can this be a bug ???

model->canFetchMore(index)

return false always....

Is there any specific reason why model not reading its row contents during the very first expansion ???

Thank You!

If this is the case you can use

void MainWindow::expand(const QModelIndex &index)
{
while(model->canFetchMore(index))
model->fetchMore(index);
    qDebug() << model->data(index,Qt::DisplayRole);
    qDebug() << model->hasChildren(index);
    qDebug() << model->hasIndex(0,0,index);
    qDebug() << model->rowCount(index) ;
}