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Do you increase the contrast in post? If yes I think that whatever the cause it's exacerbated by post processing (it's most obvious on what I would guess is the thinnest neg) so it might not be a lot to worry about.
If it's only this roll, maybe it wasn't wrapped tight enough after shooting. If you think not that, you should probably check the light seals on the camera, and if OK, check that your light source for scanning is even.
(FWIW I get a similar problem on thin negs sometimes and in my case it's because my light source is slightly uneven, I fix it with a grad filter in lightroom.)
Oh sorry, I'm a few pages late again...