shot Agfa Ortho 25 from '91 in 120 format - unfortunately the frame numbers of the back paper can be seen (must be the cosmic radiation 😀)
This is a known problem, I had it with Agfapan 400, thought it was excessive X Ray from security (went through a lot that year) -- it was about the same time, - I contacted Agfa and posted on Compuserve Photo Forum, no one ever had a satisfactory explanation, Agfa included, but I had some rolls with decent images spoiled -- on some long trip as well.
Agfa replaced the film- thanks, how about the plane ticket , etc.
Cosmic rays pass through the earth, so the ones that hit film are not affected by varying density of the backing paper.
I purchased some military surplus paper, and it had been cold stored, and marked as to when samples had been tested and passed.
Any significant supply, IMO, should be tested by processing unexposed film samples. Simple enough.
There are anti fog agents for B&W as well, some used them to up contrast a bit. Bromides and such.
You are asking a question with a lot of variables, I had a batch of paper custom coated and cut for me, and it seemed to not have a long shelf life, but I have known the airlines to say they are opening paper in a darkroom and just take it in another room and ruin it, to give it back with a smile.
I was told paper has about the same density and x ray look as Semtex, when they tore up my luggage because I had a square book, every one knows from the movies Semtex is square.
Regards, John