This problem also affects trad window cleaners as well, because the frames that fill, up fill from rain water not wfp. I've had it in the past, you wipe the frame with your cloth and the frame drips for ages over the pane of glass underneath.
The difference between trad and wfp is that with trad you have a dry piece of glass with water beads running down the middle, very noticeable, but wfp the window is already covered in water so it doesn't notice (although it will still spot).
I'd say more a design flaw of the window that wfp or trad.
Simon.