Well, for the specific sample you posted it looks to me as if the red-circled portion is under-exposed and the rest of the photo is better exposed. Anyhow, there could be a number of reasons for systematic uneven tonal rendition. One would need to know a lot more about your set-up and procedures for trying to identify what’s going on.
Check if lightroom is activating a random profile with vignetting on.
I had this issue for a long time until I realize this was messing with my scans.
My hunch would be that this due to unevenness in the scanning backlight. It could be brighter near the edge. The film rebate looks a little pale outside the circled area, and darker near it. This would suggest to me that this is happening in the scan and not on the negative. Since the falloff looks linear and not circular, I’d wager it’s the backlight and not the lens. But just semi educated guesses based on the image.
You could try taking a photo without the film and fiddle with curves/contrast to you see if the scan has a brightness difference accross the frame without the film.
A. If it was your film camera.
I think that you would see this by inspecting the actual negatives together on a larger light box, preferably one large enough to show the entire film. To me it seems unlikely but it is hard to tell from just one part of a single frame.
B. If it is your scanning setup
Again, difficult to tell from seeing one part of a single frame but photographing without a negative in position and exposing to roughly give a mid tone would be helpful, use the same aperture on your ‘scanning’ lens.
Even better photograph with a piece of blank unexposed colour negative film and do the same. Viewing the resulting ‘scan’ and increasing the contrast will highlight any potential problems.