Conclusions;
-Panels must be coated with something water repellant from the factory.
-whatever it is on there is capable of withstanding a winter, including snow, but not a washing by whatever the window cleaner used.
Ponderings;
-does the water beading off help? Or maybe even hinder?
-maybe it's not deliberate but something manufacturing related.
-does anyone apply anything to previously cleaned panels? There are some shower door protective coatings that would do similar.

Apologies for the rubbish picture