QComboBox silently (i.e. without signal QComboBox::currentIndexChanged) invalidates the current index, if QStandardItemModel::setItem is called for that index of the used model.
The attached "where.txt" shows the function stack. The silent invalidation occurs within "invalidatePersistentIndex" in QStandardItemPrivate::setModel.
The application "app" in the attached "qtbug.zip" reproduces the bug. A click on "Test" in the toolbar changes the current text in the combobox from lower case to upper case or vice versa using QStandardItemModel::setItem. Please look at the output of qDebug. You see the current index before and after "setItem" and the indexes received via signal "currentIndexChanged" (if any).