While practicing my technique yesterday I noticed that whenever my leading edge overlaps a dry spot I get a single thin line trailing from the rubber edge. I did notice a tiny piece of rubber shaving on the edge which could cause it, but I’m pretty sure both edges do it.
My guess is the rubber is rounded from hitting those stupid edges, not to mention there are split marks like it wants to be ripped in half longways
Another thing I’ve noticed is that my rubber is slightly U shaped while in channel, the rubber/channel are straight looking down the side edge, but when looking top down the ends start slowly taper farther out than the middle. Think of a deck board crown, where the opposite side goes on the glass.