Only reproducible when Windows Taskbar is on top of screen. Currently I have only tested Windows 10 since I see no way to move Windows 11 Taskbar around.

A minimal reproducer:

int main(int argc, char *argv[])
{
QApplication app(argc, argv);
auto *widget = new QWidget(nullptr);
QMetaObject::invokeMethod(widget, [widget]{
widget->setFixedHeight(100);
}, Qt::QueuedConnection);
widget->show();
return app.exec();
}

First move Taskbar to top, then run the sample, and repeat maximizing and resorting the window. From 2nd maximization, one is going to see a weird offset in y-position as shown in attached gif. And with more repetition goes on, offset grows too. The increment of offset seems to be the height of Taskbar.