Description:
------------
For ease of reference, I'm forwarding libgd bug #386[1].

TL;DR: gdImageGrayScale() misses to switch to `gdEffectReplace`
alpha blending mode before calling `gdImageSetPixel` which may
result in colored images.

[1] <https://github.com/libgd/libgd/issues/386>

Test script:
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<?php

$im = imagecreatefrompng(__DIR__ . '/wings.png');
imagefilter($im, IMG_FILTER_GRAYSCALE);
imagepng($im, __DIR__ . '/wings-out.png');


Expected result:
----------------
The generated wings-out.png should be a pure grayscale image.

Actual result:
--------------
The generated wings-out.png shows faintly purple colors.