Sometimes IMPORT pdf vector box, in view of 3D proofing may appear missing surface
Such as this imported PDF box, we can see that the S5 surface is missing
Select the S5 in the surface list, and the lines included in the surface will be highlighted
(according to experience, usually the problem, are some of the rounded corners of the place)
Enlarge that place, you can see the line cross disorder not to the same point, resulting in the formation of a small 3D surface
Reapply the "Quick 3D" feature,
3D proofing immediately normal display.