The only place to update QAbstractSpinBoxPrivate::stepModifier is the QAbstractSpinBox::changeEvent virtual method. This method will not be called if style inheriting logic is called from the QWidget constructor (in this case QWidget::changeEvent handler will be called; also, calling virtual event handlers from constructor is not a good idea). I attached a simple way to reproduce this issue.
I suggest using QCoreApplication::postEvent in QWidgetPrivate::setStyle_helper. Another way to fix is not to store step modifier as class field and query SH_SpinBox_StepModifier every time it is needed.