With metal hardware acceleration activated on an Apple Silicon Mac with Retina display, using a masked gradation curve inside a layer containing transparent areas, the transparency seems to be reduced on parts of that layer, even if the gradation curves mask doesn't should alter anything in that area.
This problem only appears, if an image is present in the background. Without having a background image, the wrong transparency alteration does not happen.
This seems to be a bug in metal hardware acceleration, because deactivating it solves the problem.