Hi. I am using crypto mattes in order to isolate an asset of my scene and export it with no background.
Here is my setup:
I am mixing the render with a transparent color and using my cryptomatte mask as factor.
When I open the image in photoshop looks empty. Only the checker background.
As a second thing: I can see the buildings in the viewer node also let pass a bit of the checkered background. So they might be a bit transparent, and they should not.