When I try to open image with transparency in Image Editor window it looks exactly like it should. But when I try to show it in Render Result image using Composite node, transparent part of the image (which has 0 alpha but it's color is white) looks wrong.
You can see that it is a little bit transparent, but much less than it should be, and also there is leakage around the blue blobs:
The aplha in this region of image is zero, but it looks like that when Blender displays it, it somehow changes to about 0.9.
I ran into this problem while trying to use cryptomatte for the first time. Is there some setting I missing, or is this a bug?