I have noticed this myself.
With some customers it has happened within the first few cleans on WFP, and other customers it happens from time to time, just seems to be some residue.
The simple cure that has always worked for me over the last 5 years is to give the bottom section an extra scrub and rinse. I think that the bottom of the window particularly on downstairs windows (which is where I had always noticed it) can be the hardest to scrub the bottom edge properly, so nowadays I always make sure that the bottom edge gets a scrub.
Alex