Lessee… glasses clean — check. No transparency (first thing I thought of, too, although in this case there's no reason it would make a difference).
As for overprint preview, that's inevitable. You can't be in separations preview mode with it turned off, and you can't turn it off without exiting separations preview. (Try it!)
The document is a book chapter, thoroughly CMYK, with certficates, blend space and celebrity endorsements to prove it. And I made this illustration grayscale. So that's out.
It's just odd, and the fact that there's no logic behind it whatsoever tickles my sensible bone, which hasn't talked to my funny bone in years.
@Bob: Appearance of Black ought not to factor in. Why would it change because one plate (with no coverage) is turned off or on in seps preview? But, for the record, one of the first things I do with a new install is change the defaults to “Accurately”. Print designers who don't do that can make embarrassing mistakes…