In XFA dynamic forms, data selected in a drop-down list is not saved when you reopen the form.
The clearItems method used in the script removes not only the selected items but also the export and default values that are associated with each item. The clearItems method does not work correctly in Acrobat 8.0 and earlier but was fixed in Acrobat 9.0.
Before calling the clearItems method, save the current value and repopulate the field. Here a sample code that can be used to achieve that:
var curValue = DropDownField1.rawValue;
DropDownField1.rawValue = curValue;