Stylesheet(QSS) not work when I use qApp.setStyleSheet(qss)

Question

// MyDialog.h //

#pragma once
#include <QProgressDialog>
#include <QString>
#include <QWidget>
class MyDialog : public QProgressDialog {

    Q_OBJECT

 public:
    explicit MyDialog(const QString &text, QWidget *parent = nullptr) : QProgressDialog(text, nullptr, 0, 0, parent) {
        qDebug() << "MyDialog()!";
    }
    virtual ~MyDialog() override {
        qDebug() << "~MyDialog()!";
    }
};

// main.cpp //

#include "MyDialog.h"
#include <QApplication>

const char *qss =
    "MyDialog {"
        "min-width: 260px; background-color: peachpuff;" // qss line 1
    "}"
    "MyDialog QLabel {"
        "font-size: 16px; color: red;" // qss line 2
    "}"
    "MyDialog QProgressBar {"
        "text-align: center; border-radius: 12px; border: 1px solid grey;" // qss line 3
    "}";

int main(int argc, char **argv) {
    QApplication qApp(argc, argv);
    qApp.setStyleSheet(qss);   // 1、set qss in QApplication instance(only). (like image1 below)

    MyDialog dlg("Loading, Please wait...");
    //dlg.setStyleSheet(qss);  // 2、set qss in MyDialog instance(only). (like image2 below)
    dlg.show();

    return qApp.exec();
}

image1, where QSS is applied on QApplication instance:

iamge2, where QSS is applied on MyDialog instance:

When I set QSS to QApplication instance，only "qss line 1" works.

When I set QSS to MyDialog instance，all 3 QSS lines work.

Why? Is there a bug with my code or with Qt framework?

How to make all three lines of QSS code work when I use: qApp.setStyleSheet(qss)?

Environment:

• Windows 10（1809）
• Qt 6.5.2（MSVC 2019 64-bit）
• CMake 3.27.1
• Ninja 1.11.1