Local Adustments: Film Grain not WYSIWYG?

I haven’t explored much the Local Adjustments tool as I don’t tend to do much local editing on my photos, but I tried using it to add film-grain-like noise to some of my photos, and I noticed the adjustments don’t seem to be reflected in the 100% preview in RT, or the output doesn’t correspond with the set parameters. Here’s an example showing RT’s 100% preview besides the generated TIFF opened in the Gimp:

For the above photo, I applied the ‘neutral’ profile, enabled the ‘Black-and-White’ module without changing anything, then added a new spot with method ‘Full image’. Then I adjusted the ‘Film grain’ parameters as shown, looking at the image at 100% to try to see the effect of each parameter as I changed its value.

I tried it on several raw photos from different cameras (Panasonic GM1, Panasonic GX1, Panasonic LX7, Canon 350D, Canon G9, Canon G3), and it seems to be consistent across all of them. Before filing a bug report, I’d like to know whether I’m overlooking anything simple, since I’m not acquainted with LA. It might even be expected behaviour, though I find it odd since it makes it difficult to set the proper adjustments in the tool, according to one’s expectations.

Kubuntu 20.04 here, and my RT is compiled locally. I’ve checked the latest commits on ‘dev’ and there doesn’t seem to be any recent changes related to this behaviour, though I can easily fetch all recent changes and recompile:

In case anyone is interested, I confirmed the problem described above also occurs on the latest Win64 ‘dev’ build. I decided to open an issue on Github:

‘Film Grain’ 1:1 preview works as expected in ART and also in Darktable (from where algorithm was ported).

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