We have checked the box for Save As Default in Foxit PhantomPDF Printer properties dialog box and hit OK button to apply the setting, but when we go back in the box is unchecked again.
The function of Save As Default option is for saving those settings you made in Foxit PhantomPDF Printer Properties dialog box into a XML file which named FoxitPrinterProfile.xml under the following directory:
C:\Users\%username%\AppData\Roaming\Foxit Software\Foxit PDF Creator\Foxit PhantomPDF Printer
Then when you call Foxit PhantomPDF printer again to print document into PDF file, Foxit PhantomPDF Printer will read those settings in FoxitPrinterProfile.xml this XML file to create PDF file accordingly.
In summary, actually the Save As Default option is equivalent of a switch for saving settings made in Foxit PhantomPDF Printer Properties dialog box into a .xml format file when you click on it, and therefore there is no need to keep the option Save As Default option checked all the time.