Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
Never mind.. Figured it out. Its from the -strip option (-thumbnail does an implicit call to this). The -strip was removing the color profile, which is CYMK..
And I guess CYMK simply stripped of the profile is a negated RGB.