I'm not sure what would cause this. Are you sure it is there when you turn on Overprint Preview?
You might find using an imposition script and exporting to PDF instead of printing to postscript through Print Booklet is more reliable. For simple 2-up saddle stitch impositions without cross-over content it's really hard to beat Dave Saunders' old script: http://pdsassoc.com/downloads/Buildbooklet.zip
If you do this a lot, and need more complexity, you might want to look at IDImposer: Overview | IDImposer