Now (suddenly, so to speak), the paper is showing yellow streaks around the edges, some of which extend quite far into the centre of the image.
I noticed this effect on much newer Foma paper (less than six months old) at the start of this year. I had put it down to old developer, but surely that would affect the whole surface more or less evenly, wouldn’t it? The same should apply if the paper were ageing; why is the yellowing only at the edges?