This is something I noticed when I was testing aiShadowMatte shader.
As I increase the areaLight sample, the fall off area of ShadowMask changes.
Is this a bug or is this something to be expected of?
[Initial setup: sphere, plane and areaLight]
[aiShadowMatte applied to the plane]
[The areaLight settings ]
[As light sample gets increased, the shadow fall off changes]