When using a QDateEdit widget with calendarPopup set to true, if the year is changed using the QCalendarWidget (once the popup is activated) adds a comma "," to the year.  It appears that when the title area of the control is updated the year is treated as a 4 digit number that is formatted based on the current locale group seperator.

Examples:

Mac 

Linux

Windows

It seems that the following does provide a workaround:

QDateEdit* dateEdit = new QDateEdit;
auto calendarWidget = dateEdit->calendarWidget();
auto locale = calendarWidget->locale();
locale.setNumberOptions(locale.numberOptions() | QLocale::NumberOption::OmitGroupSeperator);
calendarWidget->setLocale(locale);

From the QtDocs for QLocale:

It does appear from a quick review of the Qt code that QLocale::toString() is used quite often in QCalendarWidget.

I was able to reproduce this behavior using the calendar widget example (https://code.qt.io/cgit/qt/qtbase.git/tree/examples/widgets/widgets/calendarwidget?h=5.15) by modifying Window::createDatesGroupBox() and calling the setCalendarPopup(true) for the minimumDateEdit, currentDateEdit, and maximumDateEdit widgets.  Then running the example application, activating one of the 3 calendar popups and changing the year.