How to prevent Imagick from incorrectly darkening some images - php

Question

I'm having a weird imagick issue that I can't seem to diagnose.
I have the following source image:
And the following code:
$img = new Imagick($path);
header("Cache-Control: private, max-age=10800, pre-check=10800");
header("Pragma: private");
header("Expires: " . date(DATE_RFC822,strtotime(" 11 months")));
header('Content-type: image/png');
echo $img;
This results in the following image:
This only happens with a very small subset of the large library of similar icon images I have, and I cannot figure out why. Googling around I've found that some ImageMagick versions have issues with weird color profiles, but they all have the same color profile (sRGB), they're all the same dimensions, and while this image is greyscale it does not only happen to greyscale images, etc.
Even curiouser, it does not happen at all on my staging server, which runs the same the same version of php5-imagick (originally 3.2.1 but upgraded to 3.3.0RC2 in the hopes of making this problem go away). The only relevant difference I can find between the two servers is that when running phpinfo, the staging (unaffected) server says
Imagick compiled with ImageMagick version ImageMagick 6.8.9-9 Q16 x86_64 2015-01-05 http://www.imagemagick.org
Imagick using ImageMagick library version ImageMagick 6.8.9-9 Q16 x86_64 2015-09-19 http://www.imagemagick.org
while the production (affected) server says
Imagick compiled with ImageMagick version ImageMagick 6.7.7-10 2014-03-08 Q16 http://www.imagemagick.org
Imagick using ImageMagick library version ImageMagick 6.7.7-10 2014-03-08 Q16 http://www.imagemagick.org
To try to remedy this I've compiled and installed ImageMagick 6.9.2-5 on the production server from source. This has not affected the results of phpinfo which still reports 6.7.7-10, in spite of convert -version returning
Version: ImageMagick 6.9.2-5 Q16 x86_64 2015-11-02 http://www.imagemagick.org
I can't figure out how to point the imagick extension to the new ImageMagick binaries. And futher I actually have no idea if the version of ImageMagick is what's causing this effect, but I have run out of other ideas and it's the only difference I can find, so it's the rabbit hole I've been jumping down for the last few hours.
Is there a way to configure the ImageMagick path in php5-imagick? Or has anyone else experienced this weird effect on (some) images and successfully solved it in some other way?