I have loads of houses like this , one house you cannot see out of the windows because it is that bad. But these were like that when I use to trad clean them.
The route cause is bad building deisgn, even on new houses this is happening.
On one brand new building which has no top windows I pointed it out to the owner. In Scotland it is pretty bad as each house is rendered and then stone chippings so the water running down the walls gets contaminated by the glue they use to adhere the stones. This then runs onto the windows and marks them.
WFP is not the root cause as when you do the bottom windows you will have washed the residue that has run from the upstairs windows. It's the rain and I do not know what builders are using now but nearly every new house will have these problems.
Maybe worth looking into and speaking to someone in the building trade.
Doug