The forum is reminding me that I am posting too much. 
Deleted a bunch: hope it doesn’t affect the flow.
Where does this happen in the pipeline?
Related remarks
– I used to inpaint pixels but abandoned the practice because it can make it worse.
– I wonder if you could use the raw data or embedded preview image to estimate the luminance…?
– In my experience, negative values tend to surface in the dark regions. In RT, at least for Urban Nightscape | Cape Town, they show up about saturated light at point sources (traffic and vehicle signal lights), though @heckflosse told me that he didn’t have that problem.