Bruce, the vendor was: getdoriginal. he still has more listed . description claims to have frozen since manufactured date. who knows.?
I have the SAME problem -- used to get it badly on the older 'EP2 'paper but now I have it on some fridge-stored RA4 paper as well !
This is an example of what I have long complained of. It is, indeed, a yellowing of the base and cannot be corrected unless you try to correct it somewhat with restrainer (benzotriazole, perhaps). Even then, it is corrected only somewhat. And, let's face it, the truncated development necessary for development without base density will possibly compromise the blacks and turn them into blue-blacks. How I do wish that there was a full remedy that we have for B&W paper whereby we can use Farmer's Reducer after the fix to 'bring back' the pure whites that make the print outstanding.
I did not try PE's offering but I have a feeling that this problem is NOT with the blix (I could well be wrong, though). The most frustrating problem with color paper has ALWAYS been the inability to produce absolutely pure whites unless the paper is really new. And, yes, like the OP, I also tried freezing paper (at great cost to my ability to store food) only to find out that this 'solution' was not really a solution. This slow deterioration is fully correctable with B&W paper until the B&W paper gets considerably fogged. That luxury does not exist with color paper and you cannot simply change filtration because you NEED that pure white in so many scenes. This is the MAIN reason that I limit my color work. VERY annoying. The 'coin' test is best to determine the extent of the fog: simple hold down a coin, FIRMLY, on the paper under full room light and then process. - David Lyga
Again, I have had great results with benzotriazole removing yellow for from Crystal Archive C, but not Kodak paper. In the world of black and white, up to 2g/L potassium bromide was also suggested. Would this work with color paper, or would it just screw up the colors? I have some Portra paper that I would love to be able to use - I love its contrast profile, but it has a slight yellow green fog.
Dont use bromide!
Fix a small piece of paper right out of the box. See if it is yellow. If it is, then the support has yellowed and Kodak should know as this should not happen with Endura.
Process a tiny piece in a B&W process and compare with the above, if the above is OK. Both should be white. If not, and the Endura is gray, a layer is fogged somehow by bad keeping or low level light exposure. BTAZ might help but at very low levels. PMT may help as well. Both cause speed loss and may cause crossover.
If the paper passes both of those tests, then the process is somehow at fault as the paper is fog free.
Supra Endura was current in cut sheets up until a couple years ago, and there's plenty that isn't age-fogged. The base does tend to yellow but it takes years, in my experience. Anything within the last 2-3 years should be fine. I have cut-sheet supra endura with 2012 expiration dates, I believe.