NumLock does not work by default for number entry using keypad (acts as cursor)

See minimal testcase below. In this example, I have a
QDoubleSpinBox, but the problem also shows with QSpinBox and QLineEdit, so I suspect this is a general input problem. It may not belong to the Input method components--it seemed like the most appropriate, but please reassign if not.

I start the test program with NumLock enabled on the keyboard.
It's not possible to enter numbers using the numeric keypad--it's in cursor key mode. 2 and 8 (i.e. up and down) increase and decrease the spinbox number by one, but I can't enter numbers as expected.
If I press NumLock twice (i.e. turn it off and on again) then it's possible to enter numbers using the keypad.

If I start the test program with NumLock disabled, then I get exactly the same behaviour as above.

It looks like Qt is not respecting the state of NumLock when it initially starts up, and it's defaulting the keypad to the NumLock off state (i.e. cursor keys). Number entry mode is only enabled when NumLock is next set to on. I would have expected that Qt would respect the NumLock state.

Regards,
Roger

Built with "g++ -o test test.cpp $(pkg-config --cflags --libs Qt5Core Qt5Gui Qt5Widgets) -fPIC"

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#include <QApplication>
#include <QDoubleSpinBox>
#include <QMainWindow>

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

QMainWindow w;
w.setCentralWidget(new QDoubleSpinBox(&w));
w.show();

return a.exec();
}
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