letting in a lot of light, similar to your problem.
you will know whether either the first or second curtain is the issue. If you see that smudge, you know one of your curtains have a thin spot.
This is minus density in the negative, so not a light leak or a thin shutter curtain etc.
Since it's sharply defined and minus density, I still think it's fouling of the emulsion side that's present before the film goes into the developer. The fouling possibly washes away in further processing, but lingers long enough to impede development in this spots. Maybe something greasy or waxy. It could even be something that sticks to the non-emulsion side of the film and gets imprinted on the emulsion side as the film is rolled up; something like a lubricant on the finishing equipment.
@Nikanon, is it always on the same type of film, or does the issue also occur on other brands & types?