Is this a BUG -- If not, please help!

I am using Freehand MX on a PC. My file includes a TIF image that has a transparent area, a cut out around a person's head. Looks fine in Freehand, but when I export it as a PDF, the area of the TIF that is transparent changes the background colour -- the blue becomes purple.

Has anyone found this, and is there a workaround?

Thanks,
Geoff