The subject problem occurs if the separator page file does NOT have the archive bit set.
To workaround this behavior, set the archive bit on the separator file:
1. Open a CMD.EXE window.
2. type the following command and press Enter:
attrib +a FileName
Where FileName is the fully qualified name of the separator page file, normally at %SystemRoot%\System32.
3. Add the separator page in the Printer Properties dialog.