Explanation sought :strange paper fog

I recently acquired,for a very low price,a quantity of Stirling RC VC paper.
This is old stuff,as Stirling have been out of production for years,but,as I had right of return,it was worth the punt.
Surprise the First : good D-max and clean whites.
Surprise the Second :returning to the darkroom the next day to continue, I encountered terrible fogging.
I was using Dektol 1:2,which I had stored overnight in a sealed container,topped off with some fresh stock.
Processing some unexposed paper confirmed the fogging.
As a last resort,I mixed a new working solution of Dektol,although the pre-used stuff still had plenty of activity, and the resulting prints were just fine.
Can anyone explain why the paper is so sensitive to the freshness of the developer?
-FWIW, the paper is the incorporated developer type.
The only thing I can think of is increased levels of Bromine?/Bromide? in the pre-used developer.

Perhaps you accidentally got some fixer into the Dektol bottle and it's causing fogging.

Maybe it's not. Maybe something else is going on here. I don't think it's the bromide. That would act as a restrainer, if anything.

Could it be something else that you've overlooked? I managed to fog some fresh paper in fresh, very warm (95F) Dektol once. I was scratching my head over the problem until I realized that the developer was very hot, around 95 deg. F. Once it cooled down, the problem went away.