I was able to create banding. The banding occurs when a color level is barely over the black clipping level of the algorithm. It was present in exposures from 1000 to 20000, as long as the light was adjusted to have a level (in this case red level) barely touching the black clipping. The banding is more visible with higher red gains, as @cpixip had indicated. The image below was with red gain at 2.8 and blue gain at 1.9.
I would speculate that what we have/see is noise partial clipping (bottom side), and speculate further that if there is any slight variation in the image black floor intensity, resulting in some black levels reaching the black clipping and some not, it would happen regardless of the light source. When the gain of red is increased, the noise amplitude is greater, and the effect is more visible. But the problem is apparent with all the channels -when their respective levels- reach the black clipping threshold.
In the case of @cpixip, that would be due to scene dark tones of natural light. But in the case of @verlakasalt, there may also be a variation of the illuminant which enhances the problem.
I need a bit more time to experiment… in the meantime thought that sharing the initial findings would align the collective thought process… an hopefully unconfuse us all.