Thank you both Jayanta and Dan ! Below is a the result FS image (grayscale), input raster (layer below the FS image) and imput parameters + environmental variables. How I see it is that the center point is shifted by radius-1 pixels to north-west. Output crd stystem is that of the input raster and Current Map, so no transformations are needed. The raster used in Snap and Mask coincides for sure with the input raster, and is in the same crd system as the input. The tool works fine for Radius=2, but for 3 and higher, the FS image is always shifted as radius-1.
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