In Print Organizer, when changing the printer at print time, the Paper size gets reset.
It appears this is a works as designed. It is suggested to remove the default paper size from the pltcfg. The workflow should be to set the printer then create the print definitions.
The same behavior exists with File Print, if you change the printer before you print, the paper size will get reset either to the default paper size or the last paper size listed in the pltcfg.The problem is not that you are using a default paper size, you will see the paper size change as well to the last paper size listed in the pltcfg if there is no default paper size selected when changing printers at print time.