QComboBox and model data update sometimes does not work

[Qt 5.7, Linux, C++]

Hi,

I have some QComboBox widgets which are connected to a model using a QDataWidgetMapper object. The Combobox list values are from an other model or set directly with addItem.
Using the same method (QDataWidgetMapper) for all kinds of widgets with the same model class too. This works very well so far except for the combobox.

The problem is that the model does not get a setData call when I change the combobox selection but this does not happens all the time.
So far I could not figured out what the circumstances are when this happens. Even selecting a menu element does not update the model even when the combobox shows the correct selection.
As far as I understand this, the QComboBox only updates its model when it lost the focus. I played around with the focusPolicy but without any luck.

I hope someone can give me a hint.
Thanks.