Background Masks and Wipeouts (amongst other types of 2D objects) will display with a solid fill with certain visual styles activated.
If the drawing is all 2D objects, you wouldn't even really notice that you have the wrong visual style set, except for certain objects such as this. It's happened to me plenty of times, because I use visual styles decently often. Just a thought.
but then again... you're saying this only happens when you PRINT, or on screen too?