Transparency with 8 bpp images in GUIX

I compose my display with several PNG images with transparent areas and, in GUIX, I format them as 8 bit palette with a Global/Shared palette. The problem I'm facing is that the transparent areas are not completely transparent, instead, are darkening the background. This keeps adding, so if I put an image with transparency over another image with transparency over the background, the overlapped "transparent" areas get darker and darker. I do not use any alpha blending, so I don't understand where this is coming from.

Attached are two images, one is the original, where the darkening can be seen but it's a bit difficult, and the other is a colored one where the trouble areas are highlighted with Paint in purple and green (note that being able to color them with Paint evidences that they are of different color than the surroundings, that is, not truly transparent).

It should be noted that it is not that the areas are not transparent at all, but it is really a darkening of supposedly transparent areas, because I checked that if I change the background from gray to green, I see the problem areas as increasingly darker green.