I agree that SNR should be the same for any raw converter. That is the problem!
That was resolved with having the right settings for RT. But then I found that the RT 5.8 output had problems. Which are clearly due to bugs in RT.
That’s clear because the dev version - which apparently has a new approach versus the 5.8 release version - greatly improved the situation.
I did not realize that I had resize enabled for RT. I will now rerun without it, but it would be very strange if that was the cause because the image was not actually resized.
I had the camera / lens correction enabled and CA as a hold over from previous tests. But they are also enabled for most of the other raw converters I tried - including darktable, and CaptureOne. Not for dcraw, of course because it does not support that.
I left them because I wanted to test camera SNR in a realistic setting - i.e. one that I would actually use. But it seems extremely unlikely to be the cause of the bizarre behavior of the RT output, because CaptureOne and darktable don’t have the problem, but do have lens corrections enabled.