I have not used an acid stop bath in at least 25 years and have never experienced the problem you describe. However, I do know one of the causes of this problem.

If you put the reels in PHoto FLo youwill get the increased density along the edge because Photo FLo acts as a catalyst to the developer.
In order to remove traces of Photo Flo from the reels it is necessaryto scrub them in hot water with a toothbrush.