I have some the same on a very large house. Funnily, half of them are oxidised & the other half not- goes without saying I only had problems with the oxidised ones.
Anyhow, here's what to do.
You cannot get water on the oxidised alu'. Put brush on glass & press to splay bristles. Slowly move the brush up to the top edge of the beading (there should be a thin rubber seal between glass & alu'). Only let the tips of the bristles touch this seal & move the brush left to right, kind of gently agitating the seal. That's it for the top 6" of the glass, so come down & clean the rest of the glass but don't move so aggressively that you push/splash any water onto that top bead- that must stay dry. When the rest of the glass has been scrubbed, gently do the same as before, splay the bristles, move them so they "just" touch the rubber seal & slowly move them from one corner to the other. Pull the brush down 6", lift off & rinse from there downwards as normal. Bear in mind the effects of fan jets or wind as you don't want any water being blown onto that top bead. It might seem a faff to begin with but perfect this method & you can clean any window wfp regardless of what problems a top frame might present. By splaying the bristles you are preventing water from reaching or splashing the top bead whilst also being able to clean 100% of the glass. I clean a lot of large country houses with old oxidised, knackered sash windows. Cleaning the top frame or even touching it isn't an option so I've had to perfect this technique over the last ten years!!

Hope that makes sense.