The resin beads do get past the strainer, I have found some in the fan jets, as for making the windows spot, I presume that these resin beads are already contaminated with impurities from the DI tank, and therefore transfer them to the window.
I have noticed on some windows when they sheet that you see small bits of grit on the windows which I though was dirt, maybe they are resin beads instead, and if you don't spot them and rinse them off, they instead dry onto the glass and leave marks.