You can see problen on this picture:

For get this bug, you can use project ["mdi"|http://example.com] from Qt examples.

You need add some code to main:

// code placeholder
int main(int argc, char *argv[])
{
    Q_INIT_RESOURCE(mdi);

    qApp->setAttribute(Qt::AA_UseHighDpiPixmaps);
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
//...

and change scale your system on 150%

Because of the incorect size the main menu height is incorrect

The problem is probably in qmdisubwindow.cpp in function: QSize ControlLabel::sizeHint()

The size should probably be affected by pixmap's devicePixelRatio()