changes to thumbnail handling between IM 6.49 and IM 6.5.9?

Hello all,

Our company has used ImageMagick for many years to successfully generate
thumbnails.  Since Feb 4th of this year we have used
ImageMagick-6.4.9-2-Q16-windows-dll.exe and that has worked fine.

On Tuesday I upgraded to ImageMagick-6.5.6-10-Q16-windows-dll.exe and am now
getting complaints (from multiple customers) that our thumbnails are fuzzy.

Our actual thumbnail build process is complex & uses PerlMagick but I can
demonstrate the issue using these simple commands:

convert -size 420x420 -thumbnail 420x420 -quality 75 -colorspace RGB
-sharpen 0.1x0.2 source.jpg th420_%im_version%.jpg
convert -size 140x140 -thumbnail 140x140 -quality 75 -colorspace RGB
-sharpen 0.1x0.2 th420_%im_version%.jpg th140_%im_version%.jpg

The first creates a 420px thumbnail from the source, and the second creates
a 140px thumbnail from the 420px one.

Using ImageMagick 6.4.9-2-Q16 we get this:

http://87.252.62.61/thumb/th140_64.jpg

With ImageMagick 6.5.6-10-Q16 we get this: (fuzzier, specifically with the
girl's freckles washed out)

http://87.252.62.61/thumb/th140_65.jpg

I have copied our build script and linked to the original source image and
shown both thumbs side-by-side at
http://87.252.62.61/thumb/side_by_side.html

Is anyone able to help with this?  I have searched the changes documentation
between 6.4.9 and 6.5.6 and I do not see anything related to changes to
thumbnailing or sharpening.  I haven't seen anything on this list or in the
forum related to it.  Could it be something like I need to uppercase
"-Sharpen" now or something???

I could revert to 6.4.9-2 but I hate to go back.  Also I could find new
sharpen arguments for 6.5.6, but the ones we had took weeks to decide upon
and will affect millions of images from thousands of different
photographers/cameras and so the best option would be to make what worked
before work again.

Thanks for any help!

Also this list gave me some help back in June re: color searching and I was
asked to post back what worked and what didn't.  I used the Histogram
function to get a list of which colours existed inside the image.  I used
our 140px thumbnail and used only the center 80% of it so as to reduce the
amount of data to churn.  Then I mapped all the colours into the 216-colour
web colours (by dividing each r,g,b on 0-255 by 6 to get 0..5 and then
taking $int_colour = 36 * red + 6 * green + blue).  Then using the frequency
of those 216 colours within each image, I created a database table of
mappings and built the search here:
http://theimagefile.co.uk/?Action=_SE&padv=1 .   Originally I had used 4096
colours with some proximity-matching but the database table grew too big.
So now it uses only 216 colours and requires an exact match.  I was also
concerned about JPG colour reduction where the colours of individual pixels
were different from what is seen by the naked eye -- however somehow this
hasn't been an issue, maybe because we use the smallest 140px thumbnail as a
data source or because we simplify to 216 colours.  I had experimented with
various operators to smooth out the image colours but they didn't seem to
help, so I ended up just using the raw thumbnail.