It's because that "cutting line" is already there. (from what I saw in your latest screenshot)
3 reasons which come to my mind. (there are many more)
➤ your workflow, the order you are processing/use of filters on this picture.
➤ your camera's filters/mode, firmware, lens quality
➤ excess of sharpening/contrast, and other processing
Also about what I saw on your latest screenshots(not having even a part of the original is not helping), you are trying to get back a light from what, 5-6 stops, more? It's black (yes I agree, there are informations in it), but you want to get that light back at which price? in my humble opinion, this will only deteriorate your picture unless you want to use that highly lightened photo as a layer mode or mask for something.
3 reasons which come to my mind. (there are many more)
➤ your workflow, the order you are processing/use of filters on this picture.
➤ your camera's filters/mode, firmware, lens quality
➤ excess of sharpening/contrast, and other processing
Also about what I saw on your latest screenshots(not having even a part of the original is not helping), you are trying to get back a light from what, 5-6 stops, more? It's black (yes I agree, there are informations in it), but you want to get that light back at which price? in my humble opinion, this will only deteriorate your picture unless you want to use that highly lightened photo as a layer mode or mask for something.