That’s exactly how I observed it and what I concluded as well. The histogram of the alpha channel shows a huge but narrow spike, such as (from the example crop above)
This spike contains image information, which I would guess is not a leakage effect but simply the IR light affected by the dye of the film. The scratches are almost black, to the left of the spike. This can be shown by adapting the dynamic range:
Therefore, setting the threshold to the left of the spike in the histogram works well for me.