On the left the tiff export and on the right the RT preview.
From what I can see here, the left image seems more contrasty.
So I cannot really answer your question.
Note : Under 1:1 zoom, some processings are not accurately rendered in the preview so surely the difference.
Okay, wow, finally I figured this out! Turns out, this is not related to RT, but the standard Ubuntu image viewer application (eog) displays the JPGs like that. Using Showtell, the output matches the edit, so I believe this is the “real” JPG.