Erased or deleted Backgrounds reappear after using some filters in G'MIC
I knockedout the background of an image, but it completely reappears if I apply some kinds of G'MIC-filters to it. For example the filter "Illustration Look" or "Posterized Dithering" (both category "Artistic"). Possibly these filters do not support the alpha channel, but I thought in that case the background should be filled with the background color. Seems that all color-informations of the pixels are still there, like if you use a layer mask.
In GIMPs channels dock there is an opacity channel. If you click on it's eye-symbol the background of the image will reappear also. I'm not sure if that should be that way, but it seems that it is not possible to really knockout a background.
My GIMP- version is 2.10.20-1, my OS is Windows 10 64 Bit.