QListWidgetItem is not drawn correctly - c++

Question

I build up a ruse of CustomWidgets in a QListWidget. So far, it works well.
A problem occurs when I move the vertical scroll bar downwards and want to display items that were not previously visible.
Then they are inserted as transparent rectangles, I can click them and respond to the clicks, but they are not drawn. They stay transparent!
I insert a picture to get a rough idea of the problem:
In my QListWidget I try to repaint it, but it doesn't work:
QListWidget::verticalScrollbarValueChanged(value);
auto item = this->itemAt(QPoint(24, value));
if (!item)
{
return;
}
auto widget = this->itemWidget(item);
if (!widget)
{
return;
}
// widget->resize(widget->size());
// widget->repaint();
widget->update();
What can I do? Thanks for help!
EDIT:
According to the request of Martin, I insert here a routine, which shows the addition of items:
void ListControl::AddCustomWidget(QWidget* customWidget, const QSize& size, bool forceSize)
{
if (forceSize)
{
customWidget->adjustSize();
}
auto displaySize = customWidget->size();
auto width = size.width();
auto height = size.height();
auto item = new QListWidgetItem(this);
this->addItem(item);
if (width >= 0)
{
displaySize.setWidth(width);
}
else
{
displaySize.setWidth(displaySize.width() - (this->verticalScrollBar()->width() + this->rightSpace));
}
if (height >= 0)
{
displaySize.setHeight(height);
}
item->setSizeHint(displaySize);
this->setItemWidget(item, customWidget);
}