Why are bitmaps not overprinting?

There are two overprint modes -- Mode 0 and 1.  The mode changes the bahaviour when dealing with CMYK colours.

• Mode 0 will always use the CMYK colour of the topmost object.  In other words, CMYK colours underneath will be knocked out.  Spot colours below, however, will remain and the CMYK colour will be overprinted.
• Mode 1 basically treats the C, M, Y and K channels separately and each channel works as if it were a spot colour. 

PDF always forces bitmaps to use Mode 0.  For CorelDRAW to correctly support output of overprints to PDF, CorelDRAW also forces bitmaps to use Mode 0.

Vector objects use Mode 1.

See the PDF reference manual for additional information on overprint modes.