Floor/Ceil/Round/RoundPreferFloor scale factor rounding policies behave differently in Qt 6

The text size in Qt 6 with the scale factor rounding policy set to Floor, Ceil, Round or RoundPreferFloor is different than it is in Qt 5.

Reproducer

Install Qt 5.15.2 and Qt 6.5.2 to $HOME/Qt via online installer, then extract, build and run the attached test case with both Qt 5 and 6 as follows:

unzip app.zip
cd app
cmake -DCMAKE_PREFIX_PATH="$HOME/Qt/5.15.2/gcc_64/lib/cmake;$HOME/Qt/6.5.2/gcc_64/lib/cmake" .
cmake --build .
for app in app_qt5 app_qt6; do for policy in Floor Ceil Round RoundPreferFloor PassThrough; do QT_SCREEN_SCALE_FACTORS=1.25 QT_SCALE_FACTOR_ROUNDING_POLICY=$policy ./$app & done; done

Result

Policy	Qt 5.15.2	Qt 6.5.2
Floor
Ceil
Round
RoundPreferFloor
PassThrough

As you can see, these policies have no effect on 5.15.2, despite being documented at https://doc.qt.io/qt-5/highdpi.html as having been introduced in Qt 5.14.

This can lead to Qt 6 applications looking out of place in an environment where both Qt 5 and Qt 6 applications are used.

Including the test case verbatim below as well as attached ZIP.

main.cpp

#include <QApplication>
#include <QPushButton>

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

    QPushButton button("Hello");
    button.resize(300, 50);
    button.setWindowTitle(QString("%1 - %2").arg(qVersion()).arg(QString::fromUtf8(qgetenv("QT_SCALE_FACTOR_ROUNDING_POLICY"))));
    button.show();

    return app.exec();
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.22)

project(app)

find_package(Qt6 COMPONENTS Widgets)
add_executable(app_qt6 main.cpp)
target_link_libraries(app_qt6 PRIVATE Qt6::Widgets)

find_package(Qt5 COMPONENTS Widgets)
add_executable(app_qt5 main.cpp)
target_link_libraries(app_qt5 PRIVATE Qt5::Widgets)