Unexpected color changes / image drops in PDF pictures!
Output may be affected when a PDF object with some types of transparency blending modes like lighten, screen etc. are applied by mistake, placed on a white ground. You may try these affects on illustrator by placing a white rectangle behind an image with some blending mode applied.
Original picture on PDF ▸
On Output PDF file ▸
Solution: Uncheck ‘ White Ground On PDF Page Box ’ checkbox in Layout Preferences
PreFlight check also report this error when the transparency is disabled on the layout preferences.