QwtPlotPicker does not show VLineRubberBand at mouse moving to left

[kuzulis]

Hi all.

I have faced with the "bug/feature", when I move the mouse from right to left, then the VLineRubberBand is disappeared. But when the mouse stops, then the VLineRubberBand is appeared again.
When I move the mouse to right, then the VLineRubberBand is visible always.

I use an empty plot without any curves, and pure QwtPlotPicker:

Plot::Plot( QWidget *parent ):
    QwtPlot( parent)
{
    auto tracker  = new QwtPlotPicker(this->canvas());
    tracker->setStateMachine( new QwtPickerTrackerMachine() );
    tracker->setTrackerMode( QwtPlotPicker::ActiveOnly );
    tracker->setRubberBand( QwtPlotPicker::VLineRubberBand );
}

Plot::Plot( QWidget *parent ):
    QwtPlot( parent)
{
    auto tracker  = new QwtPlotPicker(this->canvas());
    tracker->setStateMachine( new QwtPickerTrackerMachine() );
    tracker->setTrackerMode( QwtPlotPicker::ActiveOnly );
    tracker->setRubberBand( QwtPlotPicker::VLineRubberBand );
}

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and nothing more in code.

UPD: When I use the HLineRubberBand is disappeared at moving from top to bottom.
When I use the CrossRubberBand, it always stays visible.

Using QwtPlotPicker::AlwaysOn does not helps.

What is it? Bug or feature.. And, how to do the VLineRubberBand always visible?

PS: I use Qwt6.3 (multiaxes) + Qt 5.7 + MSVC2015x86, on Desktop PC in Release build target.

BR,
Denis