First off, I feel for you. This is an exasperating problem to try to solve. In my very early days of camera scanning I saw an issue with 6 x 9 medium format film that had me ready to throw the entire camera collection out the window and take up golf. Medium format film, 6 x 9, I would get rust colored stains radiating in from the tops and bottoms of the long sides of the frames. I was told it was stray light bouncing off the ceiling and lots of other suggestions that made sense but this one was none of those. The fix in this case was to mask the edges of the film, notwithstanding the fact that I had done this probably 30 times before without seeing this effect. Looking through my catalog I nailed it down to a batch of Portra 400 that I purchased in 2017. Two pro packs. Even though I regularly use a mask now, I scan my sprocket rocket as well as 110 films with the edges totally exposed to the backlight. No issues at all.
I totally believe that the light piping thing is real, Leslie Lazenby covered it in one of her blog posts at the FPP and that lady has forgotten more about film then I will ever know in 10 lifetimes. If she says it’s a thing, it’s a thing. Question is how big a thing. That comes down to the film base and manufacturing variation. Estar has wicked problems with it. So bad you will lose the first three exposures on a 35 mm roll of film if you don’t load in the dark. Even the non estar film bases use a weak dye to attenuate lateral transmission of light, in addition to the anti-halation dyes that turn your rinse water such a lovely shade of green or purple or whatever.
That said, this looks more like lens flare. The back light shining up through the sprocket holes is going to be way brighter than the light making it through the film itself. Lenses designed within the last 10 years or so benefit from truly excellent optical coating technology, things that didn’t exist 20 years ago.
The number one rule when scanning film with a digital camera is any light recorded by the camera that didn’t pass through the film first is going to cause you trouble. Reflections. Lens flare. Even subtle cases can make ugly results. You might try experimenting with different aperture settings to see if you can minimize flare or you might try borrowing a different macro lens to compare. Good luck. The problem can be eliminated, my Instagram account is loaded with sprocket rocket examples which would really show the sprocket marks if they were there.