Have a look at https://commons.wikimedia.org/wiki/File:William_Grant_Still_by_Carl_Van_Vechten.png - and the upload history. The last two should look identical to https://commons.wikimedia.org/wiki/File:William_Grant_Still_by_Carl_Van_Vechten.jpg - but seem to readjust everything randomly. Firefox displays the image correctly, so it's something to do with the thumbnail.
|Open||None||T43371 Thumbnail/imagescaler (tracking)|
|Open||None||T123381 Thumbnailer giving random changes in contrast/white point for greyscale PNG|
..confirm, I converted the image from grayscale to color (not actually adding any color), file size increased some 2.5MB and it now thumbnails correctly.
I discovered something really strange! And maybe this makes the bug easier to solve.
For example: https://upload.wikimedia.org/wikipedia/commons/thumb/archive/1/16/20160112173819%21William_Grant_Still_by_Carl_Van_Vechten.png/395px-William_Grant_Still_by_Carl_Van_Vechten.png looks too bright.
Now I download this thumbnail to my computer and open it with the image viewer on my computer. It looks normal! Now I drag this saved thumbnail to Firefox and it looks too bright again!
@Aklapper I think I know a solution! Technically workaround but okay.
https://upload.wikimedia.org/wikipedia/commons/thumb/archive/1/16/20160112174139%21William_Grant_Still_by_Carl_Van_Vechten.png/79px-William_Grant_Still_by_Carl_Van_Vechten.png looks too bright.
https://upload.wikimedia.org/wikipedia/commons/thumb/archive/1/16/20160112174139%21William_Grant_Still_by_Carl_Van_Vechten.png/79px-William_Grant_Still_by_Carl_Van_Vechten.jpg looks okay.
This means the input is okay. So if you always force color PNG output, the problem will be solved. Only downside is that color PNG files are slightly larger, but given that greyscale PNG are basically unusable as it is and people are just overwriting files with color PNGs when they encounter this, it's no real loss. Once this is done a feature request could be opened to support greyscale PNG thumbnails.