Try in any document with a listbox. Select one value, and click on "Lock" in the toolbar. Do as many selections as you want: the value locked will never change, and will not be cleared. This usually is marked in blue, as selections are in green.
I don't have the file so I may be missing something. The other option is that the field is set to have always one value selected. These fields are not cleared either. You can check the listbox to see whether the checkbox appears as ticked or not.