This is the result of uneven light distribution across the negative. You can manually improve the results working with the Vignetting sliders under the Lens Corrections panel in Lightroom, but the best results are obtined using Flat Field Correction which is fairly easy to implement as part of the digitizing process. I posted some analysis on my blog.
https://www.cuchara.photography/blog/2019/5/better-negative-scans-using-flat-field-correction-in-lightroom
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