In which galleries do they not?

In T198072#4310830, @Aklapper wrote:

In which galleries do they not?

I don't know, I haven't found any that don't do this, but I don't know how to check all the different kinds of galleries (I don't know all of the different parameters)

Ah. You wrote "at least some kinds of galleries" in the task summary so I assumed you have seen galleries that do not do this...

Which web browsers did you test this with?
Maybe the interpretation of the alpha channel in PNG files is up to the web browser's interpretation? Not sure either, but for https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/High-contrast-emblem-photos.svg/600px-High-contrast-emblem-photos.svg.png , Firefox 60 shows a grey image background on a black browser background but Chromium 67 shows a black image background on a black browser background.
On the other hand, the mobile version on https://commons.m.wikimedia.org/wiki/Commons:WikiProject:United_Nations does not show any checked background at all....