Another test would be to make flash photos in a very dark room, at night.
The issue would not show up in that situation.
If this is the case, the longer you wait before advancing the film, the denser the light leak will be on the frame. Using that premise, you can test this on a tripod. Do some shots where you advance immediately. Do others where you wait different periods of time before advancing. Do others where you cover the lens immediately after taking a shot, and then advance the film with the lens covered. But don't change the position of the camera between shots. The amount of light coming in will not be constant shot to shot if you are moving the camera. Make sure to keep track of what you did on each frame.
I had a similar problem but the opposite.
In my case the shutter curtains were out of sync...one traveled faster and one too slow than spec.
I believe the issue I had is known as shutter fade. What happens is the first curtain catches up to the second curtain that is traveling too slow so the one part of the neg is underexposed.
The first curtain travels across first, the second curtain follows. what you are describing is the first curtain slowing.
This usually shows in an uneven exposure from side to side, not a vertical band.
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