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In recent versions of ImageMagick we enhanced the fuzz factor to account for the alpha channel and in the process fixed a bug. So your command should work the same across ImageMagick versions but with a different fuzz value required for recent versions of ImageMagick.
You could try posting a URL to your image here so we can reproduce the problem. Perhaps there is still a bug in the fuzz factor we overlooked.
Its a bug in the -opaque option. ImageMagick multiplies the fuzz associated with the fill and target colors. The target is 0 so the fuzz result is 0. Will fix this bug in ImageMagick 6.2.7 Beta scheduled for release in the next week or two.