QPushButton background color always looks the same in Qt6 regardless of alpha value

Question

I am writing an application where I'm trying to apply a custom color to a QPushButton. Example code is shown below:

#include <QApplication>
#include <QMainWindow>
#include <QPushButton>

int
main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    auto* const mainWindow = new QMainWindow;

    auto* const pushButton = new QPushButton("Click me!");

    auto palette = QPalette();
    palette.setColor(QPalette::Button, QColor(255, 0, 0, 50));
    pushButton->setAutoFillBackground(true);
    pushButton->setPalette(palette);

    mainWindow->setCentralWidget(pushButton);
    mainWindow->show();

    return app.exec();
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.8)

set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

project(ExampleProject LANGUAGES CXX)

find_package(QT NAMES Qt5 COMPONENTS Widgets REQUIRED)
find_package(Qt5 COMPONENTS Widgets REQUIRED)

add_executable(ExampleProject
    ${CMAKE_CURRENT_LIST_DIR}/main.cpp
)

target_link_libraries(ExampleProject
    PRIVATE Qt::Widgets
)

In the example above, a red color is applied to the button, using a lower alpha value. When compiling the code using Qt5, the following ui is generated:

If the alpha value is adjusted, this is also visible. If the same application is compiled using Qt6, however, it looks like this:

As you can see, the button color is the desired red. However, the rendered alpha value is always the maximum value. If I try to change the alpha value in the code, it does not change the visual outcome.

Am I using the QPalette instance in the wrong way, or has something changed between Qt5 and Qt6 regarding the palette handling? Or is this maybe a bug?

I am using Qt 5.15/6.2 and KDE Plasma 5.27 as desktop environment.