I am using Cycles with denoising and filmic Color in Blender 2.79b, the compositing is done in Nuke 11.1v2 non commercial. The renderings are 32 bit linear EXR.
The shadow catcher is a large subdivided plane, the edges are extruded upwards and beveled smooth to conceal the edges. It covers the whole background.
In nuke, I experienced bright noise around the edges, as soon as I unpremultiply the footage to grade the sequence. (Viewed after premulting, of course). The RGB channel shows denoising artifacts around the edges, the alpha channel contains visible noise, even where it is supposed to be completely black. A quick workaround for me was to slightly to clamp the blackpoint of the alpha channel, but I know that this is not a viable solution.
Is this an error caused by the difference between denoised RGB and un-denoised alpha? Or am I doing something wrong elsewhere?